Honeycomb filter

ABSTRACT

A honeycomb filter includes a silicon carbide honeycomb fired body, an end face, and porous cell walls. The silicon carbide honeycomb fired body includes a plurality of cells through which exhaust gas is to flow and which include exhaust gas introduction cells and exhaust gas emission cells. The silicon carbide honeycomb fired body includes silicon carbide grains having a silicon-containing oxide layer with a thickness of 0.1 to 2 μm on surfaces of the silicon carbide grains. The end face has an aperture ratio of not less than 20% at the exhaust gas emission side. The porous cell walls define rims of the plurality of cells. The plugged portions of the exhaust gas introduction cells are arranged in vertical and horizontal lines with the porous cell walls residing between the plugged portions in the end face at the exhaust gas emission side.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 U.S.C. §119 to JapanesePatent Application No. 2013-159937, filed Jul. 31, 2013. The contents ofthis application are incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to a honeycomb filter.

2. Background Art

Particulates (hereinafter also referred to as PMs or soot) such as sootin exhaust gas discharged from internal combustion engines includingdiesel engines cause damage to environment and human bodies, and thesedays people have paid attention to this problem. Since exhaust gascontains toxic gas components such as CO, HC, and NOx, people also worryabout the influences of the toxic gas components on the environment andhuman bodies.

To overcome this problem, various filters having honeycomb structures(honeycomb filters) formed of porous ceramics such as silicon carbidehave been proposed as exhaust gas purifying apparatus. Such honeycombfilters are connected to internal combustion engines to capture PMs inexhaust gas, or to convert the toxic gas components such as CO, HC, andNOx in the exhaust gas into nontoxic gas.

For enhancing the fuel economy of internal combustion engines andavoiding troubles derived from an increase in the pressure loss duringoperation, various honeycomb filters have been proposed including thosein which the initial pressure loss is lowered by improvement of the cellstructure and those in which the rate of increase in the pressure lossis low when a certain amount of PM is accumulated.

Such filters are disclosed, for example, in JP-A 2000-218165,WO2004/024294, and U.S. Pat. No. 4,417,908.

JP-A 2000-218165 discloses a silicon carbide honeycomb filter includinga porous silicon carbide sintered body which is provided with a silicacoat for reinforcement on the inner surface of each pore such that theoxygen content of the porous silicon carbide sintered body is 1 to 10%by mass.

FIG. 18A is a perspective view schematically illustrating a honeycombfilter disclosed in WO 2004/024294. FIG. 18B is a perspective viewschematically illustrating a honeycomb fired body forming the honeycombfilter.

As shown in FIGS. 18A and 18B, WO 2004/024294 discloses a honeycombfilter 90 that includes a plurality of honeycomb fired bodies 100combined with one another with adhesive layers 105 residingtherebetween, and an periphery coat layer 106 formed on the periphery ofthe combined honeycomb fired bodies, wherein the honeycomb fired bodies100 each include exhaust gas introduction cells 102 each having an openend at an exhaust gas introduction side and a plugged end at an exhaustgas emission side, and exhaust gas emission cells 101 each having anopen end at the exhaust gas emission side and a plugged end at theexhaust gas introduction side; the exhaust gas emission cells 101 eachhave a square cross section perpendicular to the longitudinal directionof the cells; the exhaust gas introduction cells 102 each have anoctagonal cross section perpendicular to the longitudinal direction ofthe cells; and the exhaust gas emission cells 101 and the exhaust gasintroduction cells 102 are alternately (in a grid-like pattern)arranged.

Hereinafter, in the explanation of the embodiments of the presentinvention and background arts, a cell having an open end at an exhaustgas emission side and a plugged end at an exhaust gas introduction sideis simply described as an exhaust gas emission cell. Moreover, a cellhaving an open end at an exhaust gas introduction side and a plugged endat an exhaust gas emission side is simply described as an exhaust gasintroduction cell, a first exhaust gas introduction cell, or a secondexhaust gas introduction cell.

The term just described as “cell” means both of the exhaust gas emissioncell and exhaust gas introduction cell.

Moreover, a cross section perpendicular to the longitudinal direction ofcells including exhaust gas introduction cells, exhaust gas emissioncells, or the like is simply described as a cross section of the exhaustgas introduction cells, exhaust gas emission cells, or the like.

FIG. 19A is a perspective view schematically illustrating a honeycombfilter disclosed in U.S. Pat. No. 4,417,908. FIG. 19B is a viewschematically illustrating an end face of the honeycomb filter.

U.S. Pat. No. 4,417,908 discloses a honeycomb filter 110 in which allcells have the same square cross sectional shape as shown in FIGS. 19Aand 19B. In the honeycomb filter 110, exhaust gas emission cells 111each having an open end at an exhaust gas emission side and a pluggedend at an exhaust gas introduction side are adjacently surrounded fullyby exhaust gas introduction cells 112 and 114 each having an open end atthe exhaust gas introduction side and a plugged end at the exhaust gasemission side across cell walls 113. In the cross section, a side of theexhaust gas introduction cell 112 faces the exhaust gas emission cell111 across the cell wall 113, whereas the corners of the exhaust gasintroduction cells 114 respectively face the corners of the exhaust gasemission cells 111. Thus, none of the sides forming the cross sectionsof the exhaust gas introduction cells 114 faces the exhaust gas emissioncells 111.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, a honeycomb filterincludes a silicon carbide honeycomb fired body, an end face, and porouscell walls. The silicon carbide honeycomb fired body includes aplurality of cells through which exhaust gas is to flow and whichinclude exhaust gas introduction cells and exhaust gas emission cells.The exhaust gas introduction cells each have an open end at an exhaustgas introduction side and a plugged end at an exhaust gas emission side.The exhaust gas emission cells each have an open end at the exhaust gasemission side and a plugged end at the exhaust gas introduction side.The silicon carbide honeycomb fired body includes silicon carbide grainshaving a silicon-containing oxide layer with a thickness of 0.1 to 2 μmon surfaces of the silicon carbide grains. The exhaust gas introductioncells and the exhaust gas emission cells each have a uniform crosssectional shape except for a plugged portion in a cross sectionperpendicular to a longitudinal direction of the plurality of cellsthoroughly from the exhaust gas introduction side to the exhaust gasemission side. The end face has an aperture ratio of not less than 20%at the exhaust gas emission side. The porous cell walls define rims ofthe plurality of cells. The plugged portions of the exhaust gasintroduction cells are arranged in vertical and horizontal lines withthe porous cell walls residing between the plugged portions in the endface at the exhaust gas emission side.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings.

FIG. 1 is a perspective view schematically illustrating one example of ahoneycomb filter according to a first embodiment of the presentinvention.

FIG. 2A is a perspective view schematically illustrating one example ofa honeycomb fired body forming the honeycomb filter in FIG. 1. FIG. 2Bis an A-A line cross sectional view of the honeycomb fired body in FIG.2A. FIG. 2C is a view of an end face at the exhaust gas emission side ofthe honeycomb fired body in FIG. 2A.

FIG. 3A is an end face view illustrating an end of a honeycomb filteraccording to one embodiment of the present invention. FIG. 3B is anenlarged end face view illustrating an enlarged image of a part of anend face of a honeycomb filter according to one embodiment of thepresent invention.

FIGS. 4A and 4B are each a scanning electron microscope photograph (SEMphotograph) showing one example of the cross sections of cells.

FIGS. 5A and 5B are each a scanning electron microscope photograph (SEMphotograph) showing one example of the cross sectional shapes of cellsthat are different from the cells shown in FIGS. 4A and 4B.

FIGS. 6A to 6C are each an enlarged end face view illustrating anenlarged image of a part of an end face of a honeycomb filter accordingto one embodiment of the present invention.

FIG. 7 is an enlarged cross sectional view perpendicular to thelongitudinal direction of the honeycomb filter. FIG. 7 illustrates howeach cell unit (cell structure) is two-dimensionally, i.e. in X and Ydirections, repeated in the case where the second exhaust gasintroduction cells and the exhaust gas emission cells are octagonal andthe first exhaust gas introduction cells are square in the cross sectionof the cells, and also illustrates how the first exhaust gasintroduction cells and the second exhaust gas introduction cells areshared between the cell units (cell structure).

FIG. 8 is an explanatory diagram schematically illustrating a method formeasuring the initial pressure loss.

FIG. 9 is an explanatory diagram schematically illustrating a method formeasuring the pressure loss.

FIG. 10A is a graph showing a relation between PM capture amount and thepressure loss that are measured in Example 1 and Comparative Example 1.FIG. 10B is a graph showing a relation between the gas flow rate and theinitial pressure loss.

FIG. 11A is a perspective view schematically illustrating one example ofa honeycomb filter according to a second embodiment of the presentinvention. FIG. 11B is a perspective view illustrating a honeycomb firedbody forming the honeycomb filter.

FIG. 12A is a perspective view schematically illustrating one modifiedexample of a honeycomb fired body forming the honeycomb filter accordingto the second embodiment of the present invention. FIG. 12B is an endface view of the honeycomb fired body forming the honeycomb filter inFIG. 12A. FIG. 12C is a perspective view illustrating another modifiedexample of a honeycomb fired body forming the honeycomb filter accordingto the second embodiment of the present invention.

FIG. 13A is an end face view schematically illustrating one example ofthe cell arrangement at an end face of a honeycomb fired body formingthe honeycomb filter according to a third embodiment of the presentinvention. FIG. 13B is an end face view illustrating one modifiedexample of a honeycomb fired body forming the honeycomb filter accordingto the third embodiment of the present invention. FIG. 13C is an endface view illustrating another modified example of a honeycomb firedbody forming the honeycomb filter according to the third embodiment ofthe present invention.

FIG. 14 is a view of an end face at the exhaust gas emission side of thehoneycomb fired body in FIG. 13A.

FIG. 15 is an enlarged cross sectional view perpendicular to thelongitudinal direction of the honeycomb filter. FIG. 15 illustrates howeach cell unit (cell structure) is two-dimensionally, i.e. in X and Ydirections, repeated in the case where the first exhaust gasintroduction cells, the second exhaust gas introduction cells, and theexhaust gas emission cells are square in the cross section of the cells,and also illustrates how the first exhaust gas introduction cells andthe second exhaust gas introduction cells are shared between the cellunits (cell structure).

FIG. 16 is an end face view schematically illustrating one example ofthe cell arrangement in an end face of a honeycomb fired body formingthe honeycomb filter according to a fourth embodiment of the presentinvention.

FIG. 17A is an explanatory diagram schematically illustrating oneexample of a convex square cell shape. FIG. 17B is an explanatorydiagram schematically illustrating one example of a concave square cellshape. FIG. 17C is an explanatory diagram schematically illustrating oneexample of the concave square shape in which a vertex portion ischamfered. FIG. 17D is an explanatory diagram schematically illustratingone example of the convex square shape in which a vertex portion ischamfered.

FIG. 18A is a perspective view schematically illustrating a honeycombfilter disclosed in WO 2004/024294. FIG. 18B is a perspective viewschematically illustrating a honeycomb fixed body forming the honeycombfilter.

FIG. 19A is a perspective view schematically illustrating a honeycombfilter disclosed in U.S. Pat. No. 4,417,908. FIG. 19B is an end faceview schematically illustrating an end face of the honeycomb filter.

FIG. 20 is a schematic cross sectional view of silicon carbide grainshaving a silicon-containing oxide layer on the surface thereof.

FIG. 21A is a perspective view schematically illustrating a honeycombfilter according to a comparative example. FIG. 21B is a perspectiveview schematically illustrating a honeycomb fired body forming thehoneycomb filter shown in FIG. 21A.

DESCRIPTION OF THE EMBODIMENTS

FIG. 20 is a schematic cross sectional view of silicon carbide grainswith a silicon-containing oxide layer on the surface thereof.

As shown in FIG. 20, silicon carbide grains 31, which form a porousceramic product, are provided with a silica coat 32 on the surfacethereof. The silica coat 32 enlarges the necks 31 a, and widens theconnecting angle of the necks 31 a, which improves the connectingstrength of the necks 31 a.

Accordingly, the silica coat on the surface of silicon carbide grainsreinforces the necks connecting silicon carbide grains, and thiscontributes to improved strength of the sintered body as a whole.

Such honeycomb filters with improved strength are desired to achieveimproved soot mass limit as well. Conventional honeycomb filtersincrease their pressure loss as PM accumulates therein. When a certainamount of PM accumulates, such honeycomb filters should be burnt toremove PM. The term “soot mass limit” refers to the maximum amount ofaccumulated PM that does not cause thermal stress high enough to damage(e.g. crack) a honeycomb filter when burnt (the burning treatment isalso referred to as regeneration). The relationship with the thermalshock fracture resistance parameter can be represented by the formula(1).

$\begin{matrix}{{{SML} \propto R^{\prime}} = \frac{{\sigma \left( {1 - v} \right)}K}{\alpha \; E}} & (1)\end{matrix}$

(In the formula (1), R′ is the second thermal shock fracture resistanceparameter, σ is the bending strength, ν is the Poisson's ratio, α is thethermal expansion coefficient, E is the Young's modulus, and K is thethermal conductivity.)

As seen in the formula (1), the soot mass limit (SML) is known to beproportional to the second thermal shock fracture resistance parameterR′. When the soot mass limit of a honeycomb filter with a silica coat isdiscussed based on the formula (1), its higher strength (α in theformula) attributable to the silica coat increases the soot mass limitas described in JP-A 2000-218165, but the silica coat hinders heattransfer in highly thermally conductive silicon carbide, and thusreduces the thermal conductivity (K in the formula). Accordingly, theimprovement in strength does not lead to improvement in the soot masslimit.

WO 2004/024294 discloses a honeycomb filter 90 which includes exhaustgas introduction cells 102 each having a plugged end at the exhaust gasemission side. The plugged portions are combined with thin cell wallsresiding therebetween. This structure is not effective in releasing heatgenerated by the regeneration treatment through the plugs, and thus hasroom for improvement in terms of heat release performance. Besides this,the absence of a silica coat on the surface of the sintered bodyprevents improvement in the soot mass limit.

U.S. Pat. No. 4,417,908 discloses a honeycomb filter 110 that includesexhaust gas introduction cells 111 each having a plugged end at theexhaust gas emission side. Although the plugged portions are arranged invertical and horizontal lines, no silica coat is formed on the surfaceof the sintered body. Unfortunately, without improvement in the strengthof the honeycomb filter, the soot mess limit cannot be improved.

A honeycomb filter of the embodiments of the present invention includesa silicon carbide honeycomb fired body, the honeycomb fired bodyincluding a plurality of cells for allowing exhaust gas to flowtherethrough, the cells including exhaust gas introduction cells andexhaust gas emission cells, and

porous cell walls defining rims of the plurality of cells,

the exhaust gas introduction cells each having an open end at an exhaustgas introduction side and a plugged end at an exhaust gas emission side,

the exhaust gas emission cells each having an open end at the exhaustgas emission side and a plugged end at the exhaust gas introductionside,

the silicon carbide honeycomb fired body including silicon carbidegrains,

the silicon carbide grains having a silicon-containing oxide layer witha thickness of 0.1 to 2 μm on a surface thereof,

the exhaust gas introduction cells and the exhaust gas emission cellseach having a uniform cross sectional shape except for the pluggedportion in a direction perpendicular to the longitudinal direction ofthe cells thoroughly from the end at the exhaust gas introduction sideto the end at the exhaust gas emission side,

the honeycomb filter having an end face with an aperture ratio of notless than 20% at the exhaust gas emission side,

the plugged portions of the exhaust gas introduction cells beingarranged in vertical and horizontal lines with the cell walls residingtherebetween in the end face at the exhaust gas emission side.

The 0.1 to 2 μm thick silicon-containing oxide layer (silica coat) onthe surface of silicon carbide grains improves the strength of thehoneycomb filter. Additionally, the silica coat coating the surface ofthe silicon carbide grains prevents the silicon carbide grains fromreleasing SiO molecules (so-called active oxidation) when exposed to1500° C. or higher temperatures in a diesel exhaust gas atmosphere.Although the silica coat reduces the thermal conductivity of thehoneycomb filter, the plugged portions, which are arranged in verticaland horizontal lines with the cell walls residing therebetween at theexhaust gas emission side, improve the heat releasing performance of thehoneycomb filter. Thus, the improved heat releasing performance of thehoneycomb filter compensates for the reduction in the thermalconductivity caused by the silica coat, thereby increasing theregeneration limit amount of PM (soot mass limit).

When a regeneration treatment is carried out on a honeycomb filter,accumulated PM is all burnt from the exhaust gas introduction side tothe exhaust gas emission side of the honeycomb filter, while heat istransferred to the exhaust gas emission side with a flow of exhaust gas.This causes portions of the honeycomb filter closer to the exhaust gasemission end to be exposed to higher temperatures, thereby creating atemperature difference in the honeycomb filter. Due to thermal stress,the honeycomb filter develops cracks. By contrast, the honeycomb filterof the embodiments of the present invention can reduce the temperaturedifference in the end portion of the honeycomb filter at the exhaust gasemission side in the direction perpendicular to the longitudinaldirection of the cells because the sealed ends, in other words, theplugged portions of the exhaust gas introduction cells, which arearranged in vertical and horizontal lines in the end face at the exhaustgas emission side, function as both a heat conducting layer forconducting heat in the direction perpendicular to the longitudinaldirection of cells and a heat releasing layer for releasing heat to theoutside. Therefore, less thermal stress is created, and thus thehoneycomb filter of the embodiments of the present invention is moreresistant to cracks than conventional honeycomb filters when the sameamount of PM is burnt. In other words, the regeneration limit amount ofPM can be increased. Another feature is that the end face at the exhaustgas emission side has an aperture ratio of not less than 20%. This meansthat the exhaust gas emission cells constitute a large proportion of thevolume, and a large area of the cell walls of the exhaust gas emissioncells is available for contact with exhaust gas. Accordingly, more heatis released with emission of exhaust gas, while emission of exhaust gasfrom the honeycomb filter disperses gas more efficiently. Therefore,less heat remains on the exhaust gas emission side of the honeycombfilter, thereby creating a smaller temperature difference in thehoneycomb filter. As a result, the regeneration limit amount of PM canbe increased.

If the thickness of the silicon-containing oxide layer on the surface ofthe silicon carbide grains is less than 0.1 μm, the oxide layer is toothin to sufficiently improve the strength of the honeycomb filter. Onthe other hand, if the thickness of the silicon-containing oxide layeris more than 2 μm, the oxide layer is very thick, leading to a largereduction in the thermal conductivity of the honeycomb filter. Thus, theregeneration limit amount of PM cannot be increased.

If the aperture ratio of the end face at the exhaust gas emission sideis less than 20%, the exhaust gas emission cells constitute a smallproportion of the volume. This structure reduces the amount of heatreleased with emission of exhaust gas after contact with the cell wallsof the exhaust gas emission cells. Therefore, the heat releasingperformance is not improved, and thus the regeneration limit amount ofPM is less likely to be improved.

In the case of the honeycomb filter disclosed in U.S. Pat. No. 4,417,908in which the exhaust gas introduction cells and the exhaust gas emissioncells have the same cross sectional area, and are present at a ratio ofapproximately 3:1, the cell walls constitute not more than 20% of theend face at the exhaust gas emission side provided that the apertureratio of the end face is not less than 20%. This structure has very lowstrength, and therefore has a small regeneration limit amount of PM.

In the honeycomb filter of the embodiments of the present invention, theaperture ratio of the end face at the exhaust gas emission side ispreferably not less than 25%. In this case, much higher heat releasingperformance is ensured, and therefore the regeneration limit amount ofPM is further improved.

The aperture ratio of the end face at the exhaust gas emission side ispreferably not more than 40%, and more preferably not more than 35%. Alarger aperture ratio leads to higher heat releasing performances, butdisadvantageously leads to a smaller aperture ratio of the end face atthe exhaust gas introduction side. This means that only a smaller spaceis secured for accumulation of PM and ash, which is a product of PMburning, and ash will cause an increase of the pressure loss.

The aperture ratio of the end face at the exhaust gas emission sideherein refers to the proportion of the total cross sectional area of theexhaust gas emission cells to the area of the end face of the honeycombfilter.

The following will specifically demonstrate the aperture ratio based ona figure. FIG. 2C is a perspective view schematically illustrating oneexample of the end face at the exhaust gas emission side of a honeycombfixed body forming the honeycomb filter of the embodiments of thepresent invention.

The aperture ratio of the end face at the exhaust gas emission sideherein is the proportion of the total cross sectional area of theexhaust gas emission cells 11, 11A and 11B of FIG. 2C to the area of theend face of the honeycomb filter, and is determined by dividing the areaof the end face at the exhaust gas emission side of the honeycomb filterexcluding the cell walls, the plugged portions, and adhesive layers (ifpresent), by the area of the end face at the exhaust gas emission sideof the honeycomb filter, and multiplying the quotient by 100. Theaperture ratio of the honeycomb filter can be calculated from measureddimensions of the honeycomb filter and the cells, or can be measuredwith a three-dimensional measuring apparatus. As seen in FIG. 2C, theplugged portions of the exhaust gas introduction cells are arranged invertical and horizontal lines with the cell walls residing therebetweenin the end face at the exhaust gas emission side. Namely, the pluggedportions continue along the direction perpendicular or parallel to anouter wall of the honeycomb fired body. This arrangement is describedherein as being arranged “in vertical and horizontal lines”.

The silicon-containing oxide layer (silica coat) on the surface of thesilicon carbide grains can be measured by FIB-TEM observation of thecross section of the silicon carbide grains. Specifically, a 3 mm squarepiece is cut out from a cell wall, and processed by FIB into a samplefor observation of the cross section of silicon carbonate grains.Observation of the sample with TEM (transmission electron microscope:H9000NAR from Hitachi High-Technologies Corporation) at an acceleratingvoltage of 300 kV at 50000× total magnification enables measurement ofthe thickness of the silica coat in the cross section of the siliconcarbonate grains. The term “FIB” is an abbreviation of focused ion beam,and refers to a finely focused beam of ions accelerated by an electricfield. FIB processing refers to a processing technique for eroding asample which involves focused ion beam radiation for causing atoms inthe sample surface to move out therefrom (sputtering phenomenon).

The thickness of the silicon-containing oxide layer (silica coat) on thesurface of the silicon carbide grains is more preferably 0.2 to 1 μm,and still more preferably 0.25 to 0.7 μm.

The plugged portions of the exhaust gas introduction cells of thehoneycomb filter of the embodiments of the present invention have awidth of not less than 0.5 mm.

When the width is not less than 0.5 mm, the plugged portions areapparently more effective than the cell walls as a heat conducting layerand a heat releasing layer for releasing heat to the outside.

The width of the plugged portions refers to the minimum width among thevertical and horizontal widths of the plugged portions arranged invertical and horizontal lines in the end face at the exhaust gasemission side, and specifically refers to the lengths shown as La and Lbin FIG. 2C and Lc and Ld in FIG. 14. FIG. 14 is a schematic end faceview of one example of cell arrangement in the end face at the exhaustgas emission side of a honeycomb fired body forming a honeycomb filteraccording to a third embodiment of the present invention, and theplugged portions are arranged in vertical and horizontal lines in theend face at the exhaust gas emission side.

Accumulated PM in the honeycomb filter is burnt and removed by aregeneration treatment at certain intervals, as described above. Theintervals are determined based on factors such as increase of thepressure loss besides the regeneration limit amount of PM. Namely, acertain level of increase in the pressure loss should be followed by theregeneration treatment even when the amount of accumulated PM is muchsmaller than the regeneration limit amount. In order to elongate theintervals and reduce loss of fuel, the regeneration limit amount of PMshould be increased and the pressure loss should be maintained low. Inthe embodiments of the present invention, the following structure isapplied to maintain the pressure loss at low levels.

In the honeycomb filter of the embodiments of the present invention,preferably, in addition to the above structure, each exhaust gasemission cell is adjacently surrounded fully by the exhaust gasintroduction cells across the porous cell walls,

the exhaust gas introduction cells include first exhaust gasintroduction cells and second exhaust gas introduction cells each havinga larger cross sectional area than each first exhaust gas introductioncell in a direction perpendicular to the longitudinal direction of thecells,

each exhaust gas emission cell has an equal or larger cross sectionalarea than each second exhaust gas introduction cell in a directionperpendicular to the longitudinal direction of the cells, and

in the cross section perpendicular to the longitudinal direction of thecells, the exhaust gas introduction cells and the exhaust gas emissioncells are each polygonal, and

a side forming the cross sectional shape of each first exhaust gasintroduction cell faces one of the exhaust gas emission cells, a sideforming the cross sectional shape of each second exhaust gasintroduction cell faces one of the exhaust gas emission cells, and theside of the first exhaust gas introduction cell is longer then the sideof the second exhaust gas introduction cell, or

a side forming the cross sectional shape of each first exhaust gasintroduction cell faces one of the exhaust gas emission cells, and noneof the sides forming the cross sectional shape of each second exhaustgas introduction cell faces the exhaust gas emission cells.

The honeycomb filter having the above configuration may have a smallerinitial pressure loss compared to conventional honeycomb filters. Evenafter accumulation of a large amount of PMs on the cell walls, thepressure loss is less likely to increase. As a result, the pressure lossis improved over the entire period from the initial stage to afteraccumulation of PMs in close to the limit amount.

The inventors of the present application see that the pressure lossoccurs due to (a) inflow resistance caused by exhaust gas flowing intothe honeycomb filter, (b) flow-through resistance in the exhaust gasintroduction cells, (c) passage resistance in the cell walls, (d)passage resistance caused by exhaust gas upon passing through a layer ofaccumulated PMs, (e) flow-through resistance in the exhaust gas emissioncells, and (f) outflow resistance caused by exhaust gas flowing out ofthe honeycomb filter. The study of the inventors has revealed that thefactors (c), (e), and (f) are controlling factors of the initialpressure loss that occurs before accumulation of PMs, and that thefactors (a), (b), and (d) are controlling factors of the transitionalpressure loss that occurs after accumulation of a certain amount of PMs.One of the controlling factors of the initial pressure loss is not thefactor (b) flow-through resistance in the exhaust gas introduction cellsbut the factor (e) flow-through resistance in the exhaust gas emissioncalls because the aperture ratio of the honeycomb filter based on theexhaust gas emission cells is smaller than the aperture ratio of thehoneycomb filter based on the exhaust gas introduction cells. Similarly,the inventors consider the factor (f) outflow resistance caused byexhaust gas flowing out of the honeycomb filter, not the factor (a)inflow resistance caused by exhaust gas flowing into the honeycombfilter, as one of the controlling factors of the initial pressure lossbecause they suppose that the resistance due to compression of the gasis smaller than the resistance due to disturbance of emission of theexhaust gas caused by eddying flow of the gas that occurs near theemission end when the gas rapidly expands upon emission from the cells.

Since the honeycomb filter of the embodiments of the present inventionhas the exhaust gas introduction cells arranged to fully surround theexhaust gas emission cells across cell walls, there are no otheropenings (exhaust gas emission cells) from which exhaust gas can flowout around each exhaust gas emission cell on the exhaust gas emissionside. This structure is less likely to cause large eddying flow uponemission of exhaust gas. This is presumed to lower the outflowresistance of the factor (f). Moreover, since the entire area of thecell walls can be used for filtration, PMs are likely to be thinly anduniformly accumulated on the inner walls of the exhaust gas introductioncells, lowering the PM layer passage resistance of the factor (d). Thus,in the provided honeycomb filter, the pressure loss is small at theinitial stage and is lass likely to increase even after accumulation ofPMs.

The phrase “cross sectional shape of a cell” herein refers to a shapeformed by an inner cell wall of the exhaust gas emission cell, firstexhaust gas introduction cell, or second exhaust gas introduction cellin the direction perpendicular to the longitudinal direction of thecell.

The phrase “cross sectional area of a cell” herein refers to an area ofa cross sectional shape formed by an inner cell wall of each exhaust gasemission cell, first exhaust gas introduction cell, or second exhaustgas introduction cell in a cross section perpendicular to thelongitudinal direction of the cell. The term “inner cell wall” refers toa surface on the inner side of a cell among surfaces of cell wallsdefining rims of cells.

Moreover, the term “side” herein refers to a segment between vertices ofa polygon in the case where cross sectional shapes formed by inner cellwalls of the exhaust gas emission cells, the first exhaust gasintroduction cells, or the second exhaust gas introduction cells arepolygons in a direction perpendicular to the longitudinal direction ofthe cells.

Furthermore, the term “length of a side” means the length of thesegment. In the case of roundly-cornered shapes with the vertex portionsformed by curved lines (so-called chamfered shape), the length of a sidemeans the length of a straight line excluding the curved line portionsfor the following reasons.

In the case where the vertex portions are formed by curved lines, thecell walls separating the cells are thick in the curve portions, andthus the curve portions have high passage resistance. This causesexhaust gas to preferentially flow into straight line portions, and thusthe length of the straight portions needs to be controlled. Hence, it isreasonable to exclude the curve portions from consideration.

Provided that the straight portions of a polygon are hypotheticallyextended, and intersections of the hypothetical straight lines are givenas hypothetical vertices, the length of the straight portion of the sideexcluding the curve portion is preferably not less than 80% the lengthof a hypothetical side given by connecting the hypothetical vertices. Inthe case of the cell having a polygonal cross sectional shape in whichthe sides have not less than 80% the length of the hypothetical sides, amain-channel-switching effect, which is a functional effect of theembodiments of the present invention, can be achieved by adjusting thelength of the sides.

In the honeycomb filter according to the embodiments of the presentinvention, a side forming the cross sectional shape of a first exhaustgas introduction cell or a second exhaust gas introduction cell isconsidered to face an exhaust gas emission cell when the followingcondition is satisfied. Provided that, in the cross sectionperpendicular to the longitudinal direction of cells, a hypotheticalperpendicular line (hereinafter referred to as a perpendicular bisector)which bisects a side of a polygon formed by the inner cell wall of afirst exhaust gas introduction cell or a second exhaust gas introductioncell is given from the side to outside the first exhaust gasintroduction cell or the second exhaust gas introduction cell, theperpendicular bisector crosses a shape region defined by the inner cellwall of an exhaust gas emission cell which is adjacent to the firstexhaust gas introduction cell or a second exhaust gas introduction cellacross a cell wall.

In such a case, a first exhaust gas introduction cell or second exhaustgas introduction cell having a side facing an exhaust gas emission cellis considered to face the exhaust gas emission cell.

In the honeycomb filter according to the embodiments of the presentinvention, a side forming the cross sectional shape of an exhaust gasemission cell is considered to face a first exhaust gas introductioncell or a second exhaust gas introduction cell when the followingcondition is satisfied. Provided that, in the cross sectionperpendicular to the longitudinal direction of cells, a hypotheticalperpendicular line (hereinafter referred to as a perpendicular bisector)which bisects a side of a polygon formed by the inner cell wall of anexhaust gas emission cell is given from the side to outside the exhaustgas emission cell, the perpendicular bisector crosses a shape regiondefined by the inner cell wall of a first exhaust gas introduction cellor a second exhaust gas introduction cell which is adjacent to theexhaust gas emission cell across a cell wall.

In such a case, an exhaust gas emission cell having a side facing afirst exhaust gas introduction cell or a second exhaust gas introductioncell is considered to face the first exhaust gas introduction cell orthe second exhaust gas introduction cell.

Moreover, in the honeycomb filter according to the embodiments of thepresent invention, a side forming a first exhaust gas introduction cellis considered to face a second exhaust gas introduction cell when thefollowing condition is satisfied. Provided that, in the cross sectionperpendicular to the longitudinal direction of cells, a hypotheticalperpendicular line (hereinafter referred to as a perpendicular bisector)which bisects a side of a polygon formed by the inner cell wall of afirst exhaust gas introduction cell is given from the side to outsidethe first exhaust gas introduction cell, the perpendicular bisectorcrosses a shape region defined by the inner cell wall of a secondexhaust gas introduction cell which is adjacent to the first exhaust gasintroduction cell across a cell wall.

In such a case, a first exhaust gas introduction cell having a sidefacing a second exhaust gas introduction cell is considered to face thesecond exhaust gas introduction cell.

Furthermore, in the honeycomb filter according to the embodiments of thepresent invention, a side forming a second exhaust gas introduction cellis considered to face a first exhaust gas introduction cell when thefollowing condition is satisfied. Provided that, in the cross sectionperpendicular to the longitudinal direction of cells, a hypotheticalperpendicular line (hereinafter referred to as a perpendicular bisector)which bisects a side of a polygon formed by the inner cell wall of asecond exhaust gas introduction cell is given from the side to outsidethe second exhaust gas introduction cell, the perpendicular bisectorcrosses a shape region defined by the inner cell wall of a first exhaustgas introduction cell which is adjacent to the second exhaust gasintroduction cell across a cell wall.

In such a case, the second exhaust gas introduction cell having a sidefacing a first exhaust gas introduction cell is considered to face thefirst exhaust gas introduction cell.

In the honeycomb filter of the embodiments of the present invention, thethickness of a cell wall separating two cells is defined as follows.

Namely, in the cross section perpendicular to the longitudinal directionof cells, provided that a hypothetical straight tine is given whichconnects geometric centers of gravity of cross sectional figures definedby the inner cell walls of two cells, the length of a segment of thestraight line overlapping the cell wall area is defined as the thicknessof the cell wall. Although cells are void, the centers of gravity hereinrefer to geometric centers of gravity of cross sectional figures definedby inner cell walls. Thus, the center of gravity can be defined even forcross sectional figures of void such as cells.

The word “adjacent” herein is equivalent to the word “adjacent” inJapanese. The word “adjacent” is used not only for a case where exhaustgas introduction cells are arranged to face exhaust gas emission cellsacross porous cell walls, but also for a case where exhaust gasintroduction cells are not facing but are arranged diagonally to exhaustgas emission cells across porous cell walls. In Japanese, the expression“Lattices are diagonally adjacent to each other” is accepted as anexemplary expression using “adjacent”.

The case where exhaust gas introduction cells and exhaust gas emissioncells each have a polygonal cross sectional shape and are arranged toface each other across porous cell walls is specifically illustrated inFIGS. 2A to 3B. In FIGS. 2A to 3B, second exhaust gas introduction cells14 face exhaust gas emission cells 11 across porous cell walls 13. In acase where exhaust gas introduction cells and exhaust gas emission cellseach have a round or elliptical cross sectional shape and a singleporous cell wall is formed by curves of the cross sectional shapes of anexhaust gas introduction cell and an exhaust gas emission cell (a casewhere a curve of the inner well of an exhaust gas emission cell and acurve of the inner wall of an exhaust gas introduction cell form thefront side and the rear side of a single cell wall in athree-dimensional view), the exhaust gas introduction cell and theexhaust gas emission cell are considered to be arranged to face eachother across a porous cell wall according to the case where exhaust gasintroduction cells and exhaust gas emission cells each have a polygonalcross sectional shape.

The case where exhaust gas introduction cells and exhaust gas emissioncells each have a polygonal cross sectional shape and are not arrangedto face each other but are arranged diagonally across porous cell wallsis specifically illustrated in FIGS. 13A to 13C. In FIGS. 13A to 13C,second exhaust gas introduction cells 44 and exhaust gas emission cells41 do not face each other and are arranged diagonally across porous cellwalls 43.

In a case where exhaust gas introduction cells and exhaust gas emissioncells each have a shape formed by carved lines, except for round orelliptical shape, an intersection of two curved lines is considered tobe a vertex and a curved line between two vertices is considered to be aside. Here, a side (curved line) of an exhaust gas emission cell orexhaust gas introduction cell is considered to face an exhaust gasintroduction cell or exhaust gas emission cell when the followingcondition is satisfied. Provided that a hypothetical perpendicular linewhich bisects a segment between vertices at the both ends of a side(curved line) forming the cross sectional shape of an exhaust gasemission cell or an exhaust gas introduction cell (A) is given from theside to the outside the exhaust gas emission cell or the exhaust gasintroduction cell (A), the perpendicular bisector crosses a shape regiondefined by the inner cell wall of an exhaust gas introduction cell or anexhaust gas emission cell (B) that is closest to the cell (A) across acell wall. Moreover, the exhaust gas emission cell or the exhaust gasintroduction cell (A) is described to face the exhaust gas introductioncell or the exhaust gas emission cell (B). In the case of the vertexportions formed by curved lines (so-called chamfered shape), the curvedlines are extended, and the hypothetical intersection of the extendedlines is considered to be a vertex.

In a case where exhaust gas introduction cells and exhaust gas emissioncells each have a shape formed by curved lines, except for round orelliptical shape, a case where exhaust gas introduction cells andexhaust gas emission cells do not face each other and are arrangeddiagonally across porous cell walls is specifically illustrated in FIG.16. In FIG. 16, second exhaust gas introduction cells 64 and exhaust gasemission cells 61 do not face each other and are arranged diagonallyacross porous cell walls 63.

The sentence “each of the exhaust gas emission cells is adjacentlysurrounded fully by the exhaust gas introduction cells across the porouscell walls” can encompass “each exhaust gas emission cell is enclosedall around by the adjacent exhaust gas introduction cells across theporous cell walls” in the present application.

Here, the phrase “arranged diagonally” refers to the arrangement inwhich exhaust gas introduction cells and exhaust gas emission cells donot face each other and satisfy the following condition. In a crosssection perpendicular to the longitudinal direction of an exhaust gasemission cell, a hypothetical segment between the geometrical center ofgravity and a vertex (in the case of chamfered vertex portions, sides(straight or curved lines) forming the cross sectional figure arehypothetically extended and an intersection of the extended lines isconsidered to be a vertex) of a cross sectional figure formed by theinner cell wall of the exhaust gas emission cell is provided. In a crosssection perpendicular to the longitudinal direction of an exhaust gasintroduction cell, a hypothetical segment between the geometrical centerof gravity and a vertex (in the case of chamfered vertex portions, sides(straight or curved lines) forming the cross sectional figure arehypothetically extended and an intersection of the extended lines isconsidered to be a vertex) of a cross sectional figure formed by theinner cell wall of the exhaust gas introduction cell is provided. Here,the provided hypothetical segments are parallel with each other oroverlapped with each other. It is to be noted that, if a pair ofhypothetical lines among plural hypothetical lines is parallel oroverlapped with each other, the other hypothetical lines may be acrosswith each other at a predetermined angle (e.g., 90°).

In the description of the tern “adjacent”, the term “exhaust gasintroduction cell” refers to both the first exhaust gas introductioncell and the second exhaust gas introduction cell.

Next, a side which faces a cell and the thickness of a cell wallseparating two cells are explained below based on figures.

FIG. 3A is an end face view illustrating an end face of a honeycombfilter according to one embodiment of the present invention, and FIG. 3Bis an enlarged end face view illustrating an enlarged image of a part ofthe end face of the honeycomb filter according to one embodiment of thepresent invention.

FIGS. 3A and 3B illustrates exhaust gas emission cells 11, and firstexhaust gas introduction cells 12 and second exhaust gas introductioncells 14 surrounding the exhaust gas emission cells 11.

A side forming the cross sectional shape of a first exhaust gasintroduction cell 12 or a second exhaust gas introduction cell 14 isconsidered to face an exhaust gas emission cell 11 when the followingcondition is satisfied. Provided that, in the cross section (end face)perpendicular to the longitudinal direction of the cells shown in FIGS.3A and 3B, a hypothetical perpendicular line (hereinafter referred to asa perpendicular bisector) which bisects a side 12 a of a polygon formedby the inner cell wall of a first exhaust gas introduction cell 12 or aside 14 a of a polygon formed by the inner cell wall of a second exhaustgas introduction cell 14 is given from the side to outside the firstexhaust gas introduction cell 12 or the second exhaust gas introductioncell 14, the perpendicular bisector A or the perpendicular bisector Bcrosses a shape region (side 11 a, side 11 b) defined by the inner cellwall of an exhaust gas emission cell 11 which is adjacent to the side 12a of the first exhaust gas introduction cell 12 or the side 14 a of thesecond exhaust gas introduction cell 14 across a cell wall as shown inFIGS. 3A and 3B.

The reason why the crossing of the bisector is set as a condition forthe facing in the embodiments of the present invention is that thepassage resistance caused upon allowing exhaust gas to pass through ator around the center of the side in the length direction, i.e. at oraround the center of the cell wall separating the exhaust gasintroduction cell and the exhaust gas emission cell, represents apressure loss caused upon allowing exhaust gas to pass through theentire wall.

According to the embodiments of the present invention, in the case wherethe cross sectional shape defined by the inner cell wall of each exhaustgas emission cell, first exhaust gas introduction cell, or secondexhaust gas introduction cell is a polygon, and the vertex portions ofthe polygon are chamfered, i.e., formed by curved lines in the crosssection perpendicular to the longitudinal direction of the cells, thebisectors of respective sides are bisectors of segments excluding thecurved lines.

Moreover, in the case where the vertex portions are roundly-cornered,i.e. formed by curved lines, the curved lines are not included in thesides. In the case where the vertex portions are chamfered in the crosssectional shape, the sides forming the cross sectional shape arehypothetically extended, and intersections of the extended sides areconsidered as hypothetical vertices. Hence, the cross sectional shape istreated as a polygon.

This is based on the following intention. For manufacturing a honeycombfilter including cells having polygonal cross sectional shapes in thecross section perpendicular to the longitudinal direction of the cellsby extrusion molding, the vertex portions of the polygonal crosssectional shapes may be formed by curved lines to prevent concentrationof stress at the vertex portions. Such cross sections in which thevertex portions are formed by curved lines are considered as polygons.

The thickness of a cell wall separating two cells is defined as follows.

Namely, in the cross section perpendicular to the longitudinal directionof the cells shown in FIGS. 3A and 3B, provided that a hypothetical lineZ₁₄ is given which connects geometric centers of gravity of the crosssectional figures defined by the inner cell walls of the two cells (thecenter of gravity of the exhaust gas emission cell 11 is O₁₁, and thecenter of gravity of the second exhaust gas introduction cells is O₁₄ inFIGS. 3A and 3B), the length D of the segment of the line Z₁₄overlapping the cell wall area is defined as the thickness of the cellwall. Although cells are voids, the centers of gravity herein refer togeometric centers of gravity of cross sectional figures defined by innercell walls. Thus, the canter of gravity can be defined even for crosssectional figures of voids such as cells.

The thickness of the cell wall is defined as above for the followingreasons. The passage resistance caused upon allowing gas to pass throughthe cell walls is the highest at a portion of the cell wall where thegas passes through at the highest flow rate, and the passage resistanceat the portion may represent the passage resistance of the cell wall.The flow rate of gas in the Longitudinal direction of the honeycombfilter is the highest at positions corresponding to the geometriccenters of gravity of the cross sectional shapes defined by the innercell walls. The flow rate concentrically decreases from the center ofgravity toward the circumference of the cross sectional shape of thecells. Thus, the flow rate of gas passing through a cell wall is thehighest at an intersection of the cell wall and a line connecting thecenter of gravity of an exhaust gas introduction cell and the center ofgravity of an exhaust gas emission cell. In view of the pressure loss asdescribed above, it is reasonable to define a length D of the segment ofa portion where a straight line connecting the centers of gravityoverlaps the cell wall area as the thickness of a cell wall.

According to the embodiments of the present invention, electronmicroscope pictures are used for measurement of the length of sides andthe thickness of cell walls, and identification of the cross sectionalshapes of cells. The electron microscope pictures are taken with anelectron microscope (FE-SEM: High resolution field emission scanningelectron microscope S-4800, manufactured by Hitachi High-TechnologiesCorporation).

The electron microscope pictures are preferably those taken by theelectron microscope at a magnification of 30×. This is because, at amagnification of 30×, irregularities due to grains or pores on thesurface (inner wall) of cell walls defining the rims of cells do notdisturb identification of the cross sectional shapes of the cells,measurement of the lengths of sides, thicknesses of cell walls, andcross sectional area of the cells. Also, at a magnification of 30×, thecross sectional shapes of the cells can be identified, and the lengthsof sides, thicknesses of cell walls, and cross sectional area of thecells can be measured.

In other words, the lengths of the respective sides of the cell aremeasured using the scale of the electron microscope photographs based onthe above definitions of the length of the cells and the thickness ofthe cell walls. The cross sectional area is arithmetically determinedbased on the obtained values including the length of the cells. Ifarithmetic calculation of the cross sectional area is complicated, thecross sectional area can also be determined by cutting a square piece (asquare with a side having a scale length) corresponding to a unit areaout of the electron microscope photograph, and weighing the cut-outpiece, while separately cutting the cross section of the cell out alongthe cross sectional shape of the cell (along curved lines in the case ofa polygonal cross section with the vertex portions formed by curvedlines), and weighing the cut-out piece, and then calculating the crosssectional area of the cell based on the weight ratio.

For example, in FIG. 4A, the cross sectional shapes defined by the innerwalls of the exhaust gas emission cells and the second exhaust gasintroduction cells are octagons having the same cross sectional areafrom one another, and the cross sectional shapes defined by therespective inner walls of the first exhaust gas introduction cells aresquare (although the vertex portions have so-called roundly-corneredshape (i.e. formed by curved lines), the cross sectional shapes are eachconsidered as a square having four sides and four vertices at fourintersections of straight lines extended from the four sides forming thecross sectional shape in the embodiments of the present invention). Inthe photograph, a 500 μm scale as displayed. A square (corresponding toa unit area) having each side in a length corresponding to 500 μmm inthe photograph is cut out of the photograph, and the cut-out piece isweighed. Next, the octagon and the square are cut out of the photograph(the four vertex portions formed by curved lines in the square are cutalong the curved lines), and the cut-out pieces are weighed. The crosssectional area is calculated based on the weight ratio between thecut-out piece and the 500 μm scale square. In the case of measuring onlythe cross sectional area ratio of the cells, the area ratio can beobtained directly from the weight ratio between the octagon and thesquare.

According to the embodiments of the present invention, the measurementof the lengths of the cells, the thicknesses of the cell walls, and thecross sectional areas can be changed from the above manual measurementto an electronic measurement by scanning the electron microscopephotograph as image data, or using the image data directly output fromthe electron microscope and entering the scale of the photograph. Themanual measurement and the electronic measurement are both based on thescale of the electron microscope image, and are in accordance with thesame principle. Thus, surely no discrepancies are found between themeasurement results of the respective measurements.

For example, the electronic measurement may be performed by using animage analysis and grain size distribution measurement software(Mac-View (Version 3.5), produced by Mountech Co. Ltd.). This softwaremeasures a cross sectional area by scanning an electron microscopephotograph as image data or using the image data directly output fromthe electron microscope, entering the scale of the photograph, andspecifying the area along the inner wall of the cell. Moreover, thedistance between any two points in the image can be measured based onthe scale of the electron microscope photograph.

A photograph of the cross section of the cells is taken with theelectron microscope by cutting a filter in a direction perpendicular tothe longitudinal direction of the cells to prepare a 1 cm×1 cm×1 cmsample including the cut face and ultrasonic cleaning the cut section ofthe sample, or coating a filter with resin and cutting the coated filterin a direction perpendicular to the longitudinal direction of the cells,and then taking an electron microscope photograph of the sample. Theresin coating does not affect measurement of the lengths of the sides ofthe cells and the thicknesses of the cell walls.

FIGS. 4A and 4B are each a scanning microscope photograph showing oneexample of the shape of the cross section of cells taken with ameasuring microscope.

FIG. 4A reveals that the cross sectional shapes of the exhaust gasemission cells 11 and the second exhaust gas introduction cells 14 areeach octagonal. The cross sectional shape of the first exhaust gasintroduction cells 12 is square. The vertex portions of each firstexhaust gas introduction cell are formed by slightly curved lines;however, extension of the four sides, which are straight lines, of thefirst exhaust gas introduction cell 12 intersect at four intersectionsto form a square having the intersections as vertices. Thus, the crosssection of the cell is considered square according to the definition ofthe embodiments of the present invention.

Moreover, calculation with MAC-View (Version 3.5) reveals that the areaof the cross sectional shape (cross sectional area) of the exhaust gasemission cell 11 and the second exhaust gas introduction cell 14 is 2.14mm², and the area of the cross sectional shape (cross sectional area) ofthe first exhaust gas introduction cell 12 is 0.92 mm².

Furthermore, as shown in FIG. 4B, since the four vertex portions of thefirst exhaust gas introduction cell 12 are formed by curved lines, thelength of a side Ls facing the exhaust gas emission cell 11 among thesides forming the cross sectional shape of the first exhaust gasintroduction cell 12 is the length excluding the curved portions.Additionally, the length Lo of a side facing the exhaust gas emissioncell among the sides forming the cross sectional shape of the secondexhaust gas introduction cell 14 corresponds to the distance between thevertices of the octagon.

As described above, the lengths of the sides Ls and Lo, and the crosssectional area can be measured using the electron microscope photograph.

FIGS. 5A and 5B are each a scanning electron microscope photograph (SEMphotograph) showing one example of the cross sectional shapes of cellsthat are different from the cells shown in FIGS. 4A and 4B.

FIG. 5A shows that the cross sectional areas of the respective exhaustgas emission cell 41, the second exhaust gas introduction cell 44, andthe first exhaust gas introduction cells 42 are each in a shape in whichstraight lines hypothetically extended from the four sides having equallength perpendicularly intersect one another at intersections(vertices), and the vertex portions are formed by curved lines. Althoughthe vertex portions of the cross sectional shape of the cell are formedby curved lines, lines extended from the four straight lines formingeach cell intersect at four intersections. Supposing that theintersections are hypothetical vertices, the four distances between thevertices are the same from one another to form a square. Thus, the crosssectional shapes of the respective cells are considered square accordingto the definition of the embodiments of the present invention.

Moreover, as is understood from FIG. 5B, a perpendicular bisector of aside forming the first exhaust gas introduction cell 42 crosses theexhaust gas emission cell 41. Thus, the side forming the first exhaustgas introduction cell 42 faces the exhaust gas emission cell 41. Incontrast, a perpendicular bisector of a side forming the second exhaustgas introduction cell 44 does not intersect with the exhaust gasemission cell 41. Thus, the side forming the second exhaust gasintroduction cell 44 does not face the exhaust gas emission cell 41. Asdescribed above, whether a side forming the second exhaust gasintroduction cell 44 or the first exhaust gas introduction cell 42 facesthe exhaust gas emission cell 41 can be determined from the electronmicroscope photograph.

A convex square according to the embodiments of the present invention isa shape formed by four outwardly curved lines of the same length. Thesquare looks as if its sides bulge from the geometric center of gravityto the outside. A concave square is a shape formed by four inwardlycurved lines of the same length. The square looks as if its sidesconcave toward the geometric center of gravity.

In the cross section perpendicular to the longitudinal direction of thecells forming the honeycomb filter according to the embodiments of thepresent invention, the first exhaust gas introduction cells, the secondexhaust gas introduction cells, and the exhaust gas emission cells eachhave a uniform cross sectional shape thoroughly from the exhaust gasintroduction end to the exhaust gas emission end except for the pluggedportion. Namely, taking the first exhaust gas introduction cell as anexample, in a cross sectional view perpendicular to the longitudinaldirection, the cross sectional shape defined by the inner wall thereofhas the same shape at any part from the exhaust gas introduction end tothe exhaust gas emission end except for the plugged portion. The sameshape means a congruent shape, and excludes similar shapes. Namely, asimilar shape means a different shape. The explanation for the firstexhaust gas introduction cell also applies to the second exhaust gasintroduction cell and the exhaust gas emission cell. The pluggedportions are excluded because the cross sectional shape defined by theinner cell wall does not physically exist at the plugged portions due tothe presence of the plugs.

The honeycomb filter of the embodiments of the present invention cancomprehensively reduce the pressure loss thoroughly from the initialstage to the stage after accumulation of PMs in close to the limitamount, as compared to conventional honeycomb filters.

In consideration of the above-described pressure loss broken down torespective resistance components, the flow-through resistance and theoutflow resistance need to be reduced for reducing the initial pressureloss. Thus, the cross sectional area of the exhaust gas emission cellsneeds to be equal to or relatively larger than the cross sectional areaof the exhaust gas introduction cells for suppressing the rapidexpansion. Wide and thin accumulation of PMs is necessary for reducingthe transitional pressure loss. Thus, the cross sectional area of theexhaust gas introduction cells needs to be relatively larger than thecross sectional area of the exhaust gas emission cells.

It has been considered impossible to reduce both of the transitionalpressure loss and the initial pressure loss. The inventors of thepresent invention further studied and completed the embodiments of thepresent invention described below.

That is, exhaust gas is preferentially introduced firstly into the firstexhaust gas introduction cells when the honeycomb filter has thefollowing structures: two kinds of the exhaust gas introduction cellsincluding the exhaust gas introduction cells each having a larger crosssectional area (second exhaust gas introduction cells) and the exhaustgas introduction cells each having a smaller cross sectional area (firstexhaust gas introduction cells) are employed as the exhaust gasintroduction cells; each exhaust gas emission cell has an equal orlarger cross sectional area than each second exhaust gas introductioncell; each exhaust gas emission cell is fully surrounded by the twokinds of the exhaust gas introduction cells; and the length of the innerwall separating the first exhaust gas introduction cell and the exhaustgas emission cell is relatively longer than the length of the inner wallseparating the second exhaust gas introduction cell and the exhaust gasemission cell, or the thickness of the wall separating the first exhaustgas introduction cell and the exhaust gas emission cell is relativelysmaller than the thickness of the wall separating the second exhaust gasintroduction cell and the exhaust gas emission cell.

The wall separating the first exhaust gas introduction cell and theexhaust gas emission cell has a larger passage area (in the case ofpolygonal cells, sides are long in the cross sectional shape) or thethickness of the wall is smaller. Exhaust gas can thus pass through suchan advantageous wall so that the passage resistance of the factor (c)can be reduced. Also, the flow-through resistance of the factor (e) canbe reduced as the cross sectional area of the exhaust gas emission cellsis relatively larger than the cross sectional area of the first exhaustgas introduction cells. Namely, both of the passage resistance of thefactor (c) and the flow-through resistance of the factor (e) can bereduced, and thereby the initial pressure loss can be reduced. Afteraccumulation of a certain amount of PMs, since the cross sectional areaof the first exhaust gas introduction cells is smaller than the crosssectional area of the second exhaust gas introduction cells, an increasein the passage resistance occurs earlier in a layer of the accumulatedPMs in the first exhaust gas introduction cells. This leads to“switching” of the main flow channel of exhaust gas in a manner that alarger amount of the exhaust gas is naturally (i.e. autonomously)introduced to the second exhaust gas introduction cells. Consequently,PMs are widely and thinly accumulated in the second exhaust gasintroduction cells each having a large cross sectional area. Hence, bothof the flow-through resistance of the factor (b) ad the passageresistance of the factor (d) can be reduced so that the transitionalpressure loss can be reduced even after accumulation of PMs.

As described above, the embodiments of the present invention haveachieved a surprising effect, which has been considered impossible, ofreducing both of the transitional pressure loss and the initial pressureloss by the autonomous switching of the main flow channel.

The aforementioned effect of reducing both of the initial pressure lossand the transitional pressure loss by “switching” of the main flowchannel to which a larger amount of exhaust gas is introduced is exertedonly when all the aforementioned features work integrally. Suchstructures or effects are not disclosed in any publicly known document.

The aforementioned international application: WO 2004/024294 discloses ahoneycomb filter including exhaust gas introduction cells 102 eachhaving an octagonal cross sectional shape and exhaust gas emission cells101 each having a rectangular cross sectional shape as shown in FIG.18B. WO 2004/024294 discloses that an increase in the cross sectionalarea of the exhaust gas introduction cells 102 enables wide and thinaccumulation of PMs, and thus the transitional pressure loss can bereduced. However, for achieving the present invention in view of WO2004/024294, some of the exhaust gas emission cells 101 each having asmaller cross sectional area need to be changed to the exhaust gasintroduction cells 102, and some of the exhaust gas introduction cells102 each having a larger cross sectional area need to be changed to theexhaust gas emission cells 101. Such changes deny the inventive conceptof the invention of WO 2004/024294, i.e. increasing the cross sectionalarea of the exhaust gas introduction cells 102.

Also, as explained earlier based on FIGS. 19A and 19B, U.S. Pat. No.4,417,908 discloses a honeycomb filter which can reduce the transitionalpressure loss by increasing the number of exhaust gas introduction cellshaving the same cross sectional area to increase the total area of theexhaust gas introduction cells so that PMs are allowed to widely andthinly accumulate.

However, for achieving the present invention in view of U.S. Pat. No.4,417,908, some of the exhaust gas introduction cells need to be changedto cells having a smaller cross sectional area. Such a change reducesthe cross sectional area of the exhaust gas introduction cells, and thusdenies the inventive concept of U.S. Pat. No. 4,417,908.

The following will explain the details of the effects of the embodimentsof the present invention by exemplifying an embodiment.

FIGS. 6A to 6C are each an enlarged end face view illustrating anenlarged image of a part of an end face of the honeycomb filteraccording to one embodiment of the present invention.

As shown in FIG. 6A, in a honeycomb filter 20, each exhaust gas emissioncell 11 having an open end at an exhaust gas emission side and a pluggedend at an exhaust gas introduction side is adjacently surrounded fullyby first exhaust gas introduction cells 12 and second exhaust gasintroduction cells 14 each having an open end at the exhaust gasintroduction side and a plugged end at the exhaust gas emission sideacross porous cell walls 13.

In the cross section perpendicular to the longitudinal direction of thecells, the exhaust gas emission cell 11 has an octagonal cross sectionthat is the same as or similar to that of the exhaust gas introductioncell 102 shown in FIG. 18B, the first exhaust gas introduction cell 12has a square cross section, and the second exhaust gas introduction cell14 has an octagonal square section that is the same as that of theexhaust gas emission cell 11. The second exhaust gas introduction cell14 has a larger cross sectional area than the first exhaust gasintroduction cell 12, and the cross sectional area is the same as theexhaust gas emission cell 11. That is, the second exhaust gasintroduction cell 14 has the same cross sectional area as the exhaustgas emission call 11, and the exhaust gas emission cell 11 has a largercross sectional area than the first exhaust gas introduction cell 12.Thus, the resistance caused by flowing of exhaust gas through theexhaust gas emission cells 11 and the resistance caused by outflow ofexhaust gas to outside the filter are reduced to low levels, and therebythe pressure loss can be reduced to a low level.

Moreover, a side 12 a facing the exhaust gas emission cell 11 among thesides twining the cross sectional shape of the first exhaust gasintroduction cell 12 is longer than a side 14 a facing the exhaust gasemission cell 11 among the sides forming the cross sectional shape ofthe second exhaust gas introduction cell 14.

Exhaust gas flowing toward the honeycomb filter 20 flows into the firstexhaust gas introduction cells 12 each having an open end at the exhaustgas introduction side and the second exhaust gas introduction cells 14each having an open end at the exhaust gas introduction side. Theexhaust gas flows in the filter in order from a part allowing easierflow and then evenly flows in the entire filter. In the honeycomb filteraccording to the embodiments of the present invention, the length (Ls)of the side 12 a of the first exhaust gas introduction cell 12 is largerthan the length (Lo) of the side 14 a of the second exhaust gasintroduction cell 14.

Thus, the surface area of a cell wall 13 a separating the exhaust gasemission cell 11 and the first exhaust gas introduction cell 12 islarger than the surface area of a cell wall 13 b separating the exhaustgas emission cell 11 and the second exhaust gas introduction cell 14,leading to easier exhaust gas passage through the cell wall 13 a.Consequently, PMs accumulate on the cell walls 13 a at an initial stage.

As described above, both of the flow-through resistance of the exhaustgas emission cells and the outflow resistance upon emission of exhaustgas from the honeycomb filter can be reduced. Thus, the Initial pressureloss before accumulation of PMs can be reduced.

The relation between the length of a side forming a cell and the surfacearea mentioned above is attributed to the following reasons.

The surface area of the cell wall 13 a separating the exhaust gasemission cell 11 and the first exhaust gas introduction cell 12corresponds to the surface area of the inner wall of the first exhaustgas introduction cell 12. The surface area of the inner wall of thefirst exhaust gas introduction cell 12 is expressed as Ls×Le, where Lerepresents an effective length of the filter excluding the length of theplugged portions at the introduction side from the exhaust gasintroduction end and at the emission side from the exhaust gas emissionend (see FIG. 2B). Similarly, the surface area of the cell wall 13 bseparating the exhaust gas emission cell 11 and the second exhaust gasintroduction cell 14 corresponds to the surface area of the inner wallof the second exhaust gas introduction cell 14. The surface area of theinner wall of the second exhaust gas introduction cell 14 is expressedas Lo×Le, where Le represents an effective length of the filterexcluding the length of the plugged portions at the introduction sidefrom the exhaust gas introduction end and at the emission side from theexhaust gas emission end. The effective length of the filter is definedas a length measured from the tip of a plug in FIG. 2B.

Thus, if the length (Ls) of the side 12 a is larger than the length (Lo)of the side 14 a, the surface area value of Ls×Le is relatively largerthan the surface area value of Lo×Le. Namely, the length of the side isequivalent to the surface area. Thus, if the length (Ls) of the side 12a of the first exhaust gas introduction cell 12 is larger than thelength (Lo) of the side 14 a of the second exhaust gas introduction cell14, the surface area of the cell wall 13 a separating the exhaust gasemission cell 11 and the first exhaust gas introduction cell 12 islarger than the surface area of the cell wall 13 b separating theexhaust gas emission cell 11 and the second exhaust gas introductioncell 14.

In FIGS. 6A to 6C, insertions relating to the effects are only partiallyillustrated. The same is true to FIGS. 3A and 3B.

Next, as shown in FIG. 6B, when a certain amount of PMs are accumulatedon the cell walls 13 a corresponding to the inner walls of the firstexhaust gas introduction cells 12, an accumulated layer of the PMs getsthick due to the small cross sectional area of the first exhaust gasintroduction cells 12. Consequently, resistance due to the accumulationof PMs increases to make the passage of exhaust gas through the cellwalls 13 a difficult. Under such conditions, exhaust gas turns to passthrough the cell walls 13 b separating the exhaust gas emission cells 11and the second exhaust gas introduction cells 14 (switching of the mainchannel). Then, PMs are also accumulated on the surfaces of the cellwalls 13 corresponding to the surfaces of the inner walls of the secondexhaust gas introduction cells 14.

Subsequently, since exhaust gas can considerably freely pass through thecell walls, exhaust gas passes through inside the cell walls 13 cseparating the first exhaust gas introduction cells 12 and the secondexhaust gas introduction cells 14 to flow into the exhaust gas emissioncells 11 as shown in FIG. 6C. In this situation, exhaust gas enters thecell walls 13 c from the first exhaust gas introduction cells 12 as wellas from the second exhaust gas introduction cells 14.

As described above, PMs accumulate on the entire surfaces of the cellwalls 13 a and 13 c around the first exhaust gas introduction cells 12corresponding the inner walls of the first exhaust gas introductioncells 12, and gradually accumulate rather more widely and thinly in alarger amount on the entire surfaces of the cell walls 13 b and 13 caround the second exhaust gas introduction cells 14 corresponding to theinner walls of the second exhaust gas introduction cells 14. The firstexhaust gas introduction cells 12 each have a smaller cross sectionalarea than each second exhaust gas introduction cell 14. Thus, in thefirst exhaust gas introduction calls 12, PMs accumulate in a thick layerwhere the passage resistance is high. For this reason, the introducedexhaust gas flows more easily into the second exhaust gas introductioncells 14 than into the first exhaust gas introduction cells 12 at anearly stage (aforementioned switching of the main channel of exhaustgas), causing the aforementioned shift of the PM accumulation.Consequently, PMs accumulate more on the entire surfaces of the cellwalls 13 b and 13 c around the second exhaust gas introduction cells 14corresponding to the inner walls of the second exhaust gas introductioncells 14 rather than the surfaces of the cell walls 13 a and 13 c aroundthe first exhaust gas introduction cells 12 corresponding to the innerwalls of the first exhaust gas introduction cells 12. It is thuspossible to make use of the entire surfaces of the cell walls 13 b and13 c around the second exhaust gas introduction cells 14 correspondingto the inner walls of the second exhaust gas introduction cells 14 foraccumulation of PMs earlier. Since the surface area of the cell walls 13b and 13 c around the second exhaust gas introduction cells 14corresponding to the inner walls of the second exhaust gas introductioncells 14 is larger than the surface area of the cell walls 13 a and 13 caround the first exhaust gas introduction cells 12 corresponding to theinner walls of the first exhaust gas introduction cells 12, when PMsaccumulate on the entire peripheries of the cell walls 13 b and 13 csurrounding the second exhaust gas introduction cells 14, theaccumulated layer is thin. Thus, the pressure loss due to exhaust gasincreases at a low rate even after accumulation of PMs. Hence, asurprisingly excellent effect of maintaining the pressure loss at a lowlevel can be achieved even though the amount of accumulated PMsincreases.

As a result, vehicles carrying the honeycomb filter according to theembodiments of the present invention do not cause a disadvantageousphenomenon for driving that are derived from an increase in the pressureloss throughout the use area, and also have good fuel economy.

The honeycomb filter of the embodiments of the present inventionpreferably has the following structure. In the cross sectionperpendicular to the longitudinal direction of an exhaust gas emissioncell and a first exhaust gas introduction cell which have polygonalcross sectional shapes and are adjacent to each other, a side, among thesides forming the cross sectional shape of the exhaust gas emissioncell, which is adjacent to the first exhaust gas introduction cellacross the cell wall in a manner facing the first exhaust gasintroduction cell is parallel to a side, among the sides forming thecross sectional shape of the first exhaust gas introduction cell, whichis adjacent to the exhaust gas emission cell across the cell wall in amanner facing the exhaust gas emission cell.

This indicates that the thickness is uniform at any part of the wallsseparating the exhaust gas emission cells and the first exhaust gasintroduction cells. Thus, it is possible to achieve high fracturestrength of the filter, easy passage of exhaust gas, and uniformaccumulation of PMs, so that the pressure loss is reduced.

In the case where the vertex portions of the polygonal cross section areformed by curved lines, the curve portions are not considered as sidesbecause naturally such portion s do not form parallel lines.

Provided that, in the cross section perpendicular to the longitudinaldirection of the cells, straight portions considered as sides arehypothetically extended, and intersections of the hypothetical straightlines are given as hypothetical vertices, the length of each side of thecross sectional shape excluding the curve portion is preferably not lessthan 80% the length of a hypothetical side of a polygon that is formedby connecting the hypothetic vertices. To put it the other way around,the length of the portion not considered as a side is preferably lessthan 20% the length of the hypothetical side.

In the case of the cell having a polygonal cross sectional shape, if thelengths of the sides are not less than 80% the respective lengths of thehypothetical sides, the main-channel-switching effect, which is aneffect of the embodiments of the present invention, can be achieved bycontrolling the length of the sides.

The honeycomb filter of the embodiments of the present inventionpreferably has the following structure. In the cross sectionperpendicular to the longitudinal direction of an exhaust gas emissioncell and a second exhaust gas introduction cell which is adjacent to theexhaust gas emission cell across a cell wall, in the case where thecells have polygonal cross sections, a side, among the sides forming thecross sectional shape of the exhaust gas emission cell, which isadjacent to the second exhaust gas introduction cell across the cellwall in a manner facing the second exhaust gas introduction cell isparallel to a side, among the sides forming the cross sectional shape ofthe second exhaust gas introduction cell, which is adjacent to theexhaust gas emission cell across the cell wall in a manner facing theexhaust gas emission cell.

This indicates that the thickness is uniform at any part of the wallsseparating the exhaust gas emission cells and the respective secondexhaust gas introduction cells. Thus, it is possible to achieve highfracture strength of the filter, easy passage of exhaust gas, anduniform accumulation of PMs so that the pressure loss is reduced.

Meanwhile, in the case where the vertex portions of the polygonal crosssectional shape are formed by curved lines, the curve portions are notconsidered as sides because naturally such portions do not form parallellines.

Provided that, in the cross section perpendicular to the longitudinaldirection of the cells, straight portions considered as sides arehypothetically extended, and intersections of the hypothetical straightlines are given as hypothetical vertices, the length of each side of thecross sectional shape excluding the curve portion is preferably not lessthan 80% the length of a hypothetical side of a polygon that is formedby connecting the hypothetical vertices. To put it the other way around,the length of the portion not considered as a side is preferably lessthan 20% the length of the hypothetical side.

In the case of the cell having a polygonal cross sectional shape, if thelengths of the sides are not less than 80% the respective lengths of thehypothetical sides, the main-channel-switching effect, which is aneffect of the embodiments of the present invention, can be achieved bycontrolling the length of the sides.

The honeycomb filter of the embodiments of the present inventionpreferably has the following structure. In the cross sectionperpendicular to the longitudinal direction of a first exhaust gasintroduction cell and a second exhaust gas introduction cell which isadjacent to the first exhaust gas introduction cell across a cell wall,in the case where the cells have polygonal cross sectional shapes, aside, among the sides forming the cross sectional shape of the firstexhaust gas introduction cell, which is adjacent to the second exhaustgas introduction cell across the cell wall in a manner facing the secondexhaust gas introduction call, is parallel to a side, among the sidesforming the cross sectional shape of the second exhaust gas introductioncell, which is adjacent to the first exhaust gas introduction cellacross the cell wall in a manner facing the first exhaust gasintroduction cell.

This indicates that the thickness is uniform at any part of the wallsseparating the first exhaust gas introduction cells and the secondexhaust gas introduction cells. Thus, it is possible to achieve highfracture strength of the honeycomb filter, easy exhaust gas passagethrough the walls from the second exhaust gas introduction cells to theexhaust gas emission cells, and wide, thin and uniform accumulation ofPMs on the inner cell walls of the second exhaust gas introductioncells, so that low pressure loss can be achieved after accumulation ofPMs.

Meanwhile, in the case where the vertex portions of the polygonal crosssection are formed by curved lines, the curve portions are notconsidered as sides because naturally such portions do not form parallellines.

Provided that, in the cross section perpendicular to the longitudinaldirection of the cells, straight portions considered as sides arehypothetically extended, and intersections of the hypothetical straightlines are given as hypothetical vertices, the length of each side of thecross sectional shape excluding the curve portion is preferably not lessthan 80% the length of a hypothetical side of a polygon that is formedby connecting the hypothetical vertices. To put it the other way around,the length of the portion not considered as a side is preferably lessthan 20% the length of the hypothetical side.

In the case of the cell having a polygonal cross sectional shape, if thelengths of the sides are not less than 80% the respective lengths of thehypothetical sides, the main-channel-switching effect, which is aneffect of the embodiments of the present invention, can be achieved bycontrolling the length of the sides.

The honeycomb filter according to the embodiments of the presentinvention preferably has the following structure. In the case where theexhaust gas emission cell, the first exhaust gas introduction cell, andthe second exhaust gas introduction cell, which are adjacent to oneanother across cell walls, each have a polygonal cross sectional shapein the cross section perpendicular to the longitudinal direction of thecells,

(a) a side, among the sides forming the cross sectional shape of theexhaust gas emission cell, which is adjacent to the first exhaust gasintroduction cell across the cell wall in a manner facing the firstexhaust gas introduction cell, is parallel to a side, among the sidesforming the cross sectional shape of the first exhaust gas introductioncell, which is adjacent to the exhaust gas emission cell across the cellwall in a manner facing the exhaust gas emission cell,

(b) a side, among the sides forming the cross sectional shape of theexhaust gas emission cell, which is adjacent to the second exhaust gasintroduction cell across the cell wall in a manner facing the secondexhaust gas introduction cell, is parallel to a side, among the sidesforming the cross sectional shape of the second exhaust gas introductioncell, which is adjacent to the exhaust gas emission cell across the cellwall in a manner facing the exhaust gas emission cell, and

(c) a side, among the sides forming the cross sectional shape of thefirst exhaust gas introduction cell, which is adjacent to the secondexhaust gas introduction cell across the cell wall in a manner facingthe second exhaust gas introduction cell, is parallel to a side, amongthe sides forming the cross sectional shape of the second exhaust gasintroduction cell, which is adjacent to the first exhaust gasintroduction cell across the cell wall in a manner facing the firstexhaust gas introduction cell.

Moreover, in the case where the first exhaust gas introduction cell, thesecond exhaust gas introduction cell, and the exhaust gas emission celleach have a polygonal cross sectional shape in the honeycomb filteraccording to the embodiments of the present invention, the distancebetween the parallel sides in the above condition (a), the distancebetween the parallel sides in the above condition (b), and the distancebetween the parallel sides in the above condition (c) are preferably thesame in the structure simultaneously satisfying the conditions (a), (b),and (c). Here, the distance between the sides is defined as follows: ahypothetical perpendicular line is given from an arbitrary point P inone side to a point Q in the other side, and the distance between thepoint P and the point Q is defined as the distance between the parallelsides.

The honeycomb filter having the above structure has the highest fracturestrength, has excellent thermal shock resistance upon regeneration, canbest reduce the pressure loss regardless of the presence or absence ofaccumulated PMs, and can avoid thermal shock damage that occurs uponregeneration of PMs.

Meanwhile, in the case where the vertex portions of the polygonal crosssection are formed by curved lines, the curve portions are notconsidered as sides because naturally such portions do not form parallellines.

Provided that, in the cross section perpendicular to the longitudinaldirection of the cells, straight portions considered as sides arehypothetically extended, and intersections of the hypothetical straightlines are given as hypothetical vertices, the length of each side of thecross sectional shape excluding the curve portion is preferably not lessthan 80% the length of a hypothetical side of a polygon that is formedby connecting the hypothetical vertices. To put it the other way around,the length of the portion not considered as a side is preferably lessthan 20% the length of the hypothetical side.

In the case of the cell having a polygonal cross sectional shape, if thelengths of the sides are not less than 80% the respective lengths of thehypothetical sides, the main-channel-switching effect, which is aneffect of the embodiments of the present invention, can be achieved bycontrolling the length of the sides.

The honeycomb filter of the embodiments of the present invention ispreferably used to purify PMs in exhaust gas discharged from internalcombustion engines of automobiles. The honeycomb filter can reduce bothof the initial pressure loss before accumulation of PMs and thetransitional pressure loss caused by accumulation of PMs in the filter,and thereby the fuel economy of the engine can be enhanced.

The honeycomb filter of the embodiments of the present invention is mostsuitably used in automobiles whose internal combustion engines arediesel engines. The amount of PMs discharged from a diesel engine islarger than that from a gasoline engine. Thus, a demand for reducing thetransitional pressure loss caused by accumulation of PMs in the filteris higher for diesel engines than for gasoline engines.

In the case of using the honeycomb filter of the embodiments of thepresent invention to purify PMs in exhaust gas discharged from internalcombustion engines of automobiles, the honeycomb filter of theembodiments of the present invention is fixed inside an exhaust tube viaa holding material.

In the honeycomb filter according to the embodiments of the presentinvention, in the cross section perpendicular to the longitudinaldirection of the cells, each exhaust gas emission cell and each exhaustgas introduction cell preferably have a polygonal cross section, and aside forming the cross sectional shape of each first exhaust gasintroduction cell faces one of the exhaust gas emission cells, a sideforming the cross sectional shape of each second exhaust gasintroduction cell faces one of the exhaust gas emission cells, and thelength of the side forming the cross sectional shape of the secondexhaust gas introduction cell is not more than 0.8 times the length ofthe side forming the cross sectional shape of the first exhaust gasintroduction cell.

The honeycomb filter having the above structure enables easier passageof exhaust gas through the cell walls separating the exhaust gasemission cells and the first exhaust gas introduction cells, effectivesuppression of the initial pressure loss, and prevention of an increasein the rate of increase of the pressure loss after accumulation of PMs.

If the ratio of the length of the side of the second exhaust gasintroduction cell to the length of the side of the first exhaust gasintroduction cell exceeds 0.8, the two sides do not have a bigdifference in length. Consequently, the initial pressure loss is hardlysuppressed.

In the honeycomb filter according to the embodiments of the presentinvention, in the direction perpendicular to the longitudinal directionof the cells, preferably, the exhaust gas emission cells are eachoctagonal, the first exhaust gas introduction cells are each square, andthe second exhaust gas introduction cells are each octagonal.

The honeycomb filter having the above structure has the same shape asthe honeycomb filter in FIGS. 6A to 6C that is described concerning theeffects thereof. Thus, the honeycomb filter can effectively suppress theinitial pressure loss, have a large surface area for allowing PMs toaccumulate thereon, and can maintain the pressure loss at a low level.

In the honeycomb filter according to the embodiments of the presentinvention, in the cross section perpendicular to the longitudinaldirection of the cells,

preferably, the cross sectional area of each second exhaust gasintroduction cell is equal in size to the cross sectional area of eachexhaust gas emission cell, and

the cross sectional area of each first exhaust gas introduction cell is20 to 50% the size of the cross sectional area of each second exhaustgas introduction cell.

The honeycomb structure having the above structure can providedifference between the resistance caused upon flowing of exhaust gasthrough the first exhaust gas introduction cells and the resistancecaused upon flowing of exhaust gas through the second exhaust gasintroduction cells, thereby enabling effective suppression of thepressure loss. If the cross sectional area of the first exhaust gasintroduction cells is less than 20% the size of the cross sectional areaof the second exhaust gas introduction cells, the cross sectional areaof the first exhaust gas introduction cells is too small. Consequently,high flow-through resistance occurs upon flowing of exhaust gas throughthe first exhaust gas introduction cells so that the pressure loss tendsto be high. If the cross sectional area of the first exhaust gasintroduction cells is more than 50% the size of the cross sectional areaof the second exhaust gas introduction cells, the difference in theflow-through resistance of the first exhaust gas introduction cells andthe flow-through resistance of the second exhaust gas introduction cellsis small. Thus, the pressure loss is hardly reduced.

In the honeycomb filter according to the embodiments of the presentinvention, the cell walls defining rims of the cells preferably have auniform thickness in any part of the honeycomb filter.

The honeycomb filter having this structure can exert the aforementionedeffects through its entire body.

In the honeycomb filter according to the embodiments of the presentinvention, in the direction perpendicular to the longitudinal directionof the cells,

preferably the exhaust gas emission cells have octagonal cross sections,the first exhaust gas introduction cells have square cross sections, andthe second exhaust gas introduction cells have octagonal cross sections,

the cross sectional shape of each second exhaust gas introduction cellis congruent with the cross sectional shape of each exhaust gas emissioncell, and

the exhaust gas emission cells, the first exhaust gas introductioncells, and the second exhaust gas introduction cells are arranged in thefollowing manner:

the exhaust gas emission cells are each surrounded by alternatelyarranged four pieces of the first exhaust gas introduction cells andfour pieces of the second exhaust gas introduction cells across theporous cell walls;

provided that hypothetical segments connecting geometric centers ofgravity of the octagonal cross sections of the four second exhaust gasintroduction cells surrounding the exhaust gas emission cell are given,an intersection of the two segments crossing a shape region includingthe cross sectional shape of the exhaust gas emission cell is identicalwith a geometric center of gravity of the octagonal cross section of theexhaust gas emission cell; and

the four segments not crossing the shape region including the crosssectional shape of the exhaust gas emission cell form a square, andmidpoints of the respective sides of the square are identical withgeometric centers of gravity of the square cross sections of the fourfirst exhaust gas introduction cells surrounding the exhaust gasemission cell, and

a side facing the first exhaust gas introduction cell across a cell wallamong the sides forming the cross sectional shape of the exhaust gasemission cell is parallel to a side facing the exhaust gas emission cellacross the cell wall among the sides forming the cross sectional shapeof the first exhaust gas introduction cell,

a side facing the second exhaust gas introduction cell across a cellwall among the sides forming the cross sectional shape of the exhaustgas emission cell is parallel to a side facing the exhaust gas emissioncell across the cell wall among the sides forming the cross sectionalshape of the second exhaust gas introduction cell,

a side facing the second exhaust gas introduction cell across a cellwall among the sides forming the cross sectional shape of the firstexhaust gas introduction cell is parallel to a side facing the fleetexhaust gas introduction cell across the cell wall among the sidesforming the cross sectional shape of the second exhaust gas introductioncell, and distances between the parallel sides are the same.

In the honeycomb filter according to the embodiments of the presentinvention, in the direction perpendicular to the longitudinal directionof the cells, the exhaust gas emission cells, the first exhaust gasintroduction cells, and the second exhaust gas introduction cells allpreferably have a square cross section.

Even in the case where the first exhaust gas introduction cells and thesecond exhaust gas introduction cells all have a square cross section,relations concerning the size position, or the like of the exhaust gasemission cells, the first exhaust gas introduction cells, and the secondexhaust gas introduction cells are different. For example, since thecross sectional area of each first exhaust gas introduction cell issmaller than the cross sectional area of each exhaust gas emission cell,the honeycomb filter of the embodiments of the present invention isdifferent from the honeycomb filter 110 (see FIGS. 19A and 19B) in theaforementioned background art, and can exert the aforementioned effectsof the embodiments of the present invention.

In the honeycomb filter according to the embodiments of the presentinvention, in the direction perpendicular to the longitudinal directionof the cells, preferably the cross sectional area of each second exhaustgas introduction cell is equal in size to the cross sectional area ofeach exhaust gas emission cell, and

the cross sectional area of each first exhaust gas introduction cell is20 to 50% the size of the cross sectional area of each second exhaustgas introduction cell.

The honeycomb filter having the above structure can provide differencebetween the resistance caused upon flowing of exhaust gas through thefirst exhaust gas introduction cells and the resistance caused uponflowing of exhaust gas through the second exhaust gas introductioncells, thereby enabling effective suppression of the pressure loss.

If the cross sectional area of the first exhaust gas introduction cellsis less than 20% the size of the cross sectional area of the secondexhaust gas introduction cells, the cross sectional area of the firstexhaust gas introduction cells is too small. Consequently, highflow-through resistance occurs upon flowing of exhaust gas through thefirst exhaust gas introduction cells so that the pressure loss tends tobe high. If the cross sectional area of the first exhaust gasintroduction cells is more than 50% the size of the cross sectional areaof the second exhaust gas introduction cells, the difference inflow-through resistance between the first exhaust gas introduction cellsand the second exhaust gas introduction cells is small. Thus, thepressure loss is hardly reduced.

In the honeycomb filter according to the embodiments of the presentinvention, in the direction perpendicular to the longitudinal directionof the cells,

preferably, the exhaust gas emission cells have a square cross section,the first exhaust gas introduction cells have a square cross section,and the second exhaust gas introduction cells have a square crosssection,

the cross sectional shape of each second exhaust gas introduction cellis congruent with the cross sectional shape of each exhaust gas emissioncell,

the exhaust gas emission cells, the first exhaust gas introductioncells, and the second exhaust gas introduction cells are arranged in thefollowing manner:

the exhaust gas emission cells are each surrounded by alternatelyarranged four pieces of the first exhaust gas introduction cells andfour pieces of the second exhaust gas introduction cells across theporous cell walls;

provided that hypothetical segments connecting geometric centers ofgravity of the square cross sections of the four second exhaust gasintroduction cells surrounding the exhaust gas emission cell are given,an intersection of the two segments crossing a shape region includingthe cross sectional shape of the exhaust gas emission cell is identicalwith a geometric center of gravity of the square cross section of theexhaust gas emission cell; and

the four segments not crossing the shape region including the crosssectional shape of the exhaust gas emission cell form a square, andmidpoints of the sides of the square are identical with geometriccenters of gravity of the square cross sections of the four firstexhaust gas introduction cells surrounding the exhaust gas emissioncell, and

a side facing the first exhaust gas introduction cell across a cell wallamong the sides forming the cross sectional shape of the exhaust gasemission cell is parallel to a side facing the exhaust gas emission cellacross the cell wall among the sides forming the cross sectional shapeof the first exhaust gas introduction cell,

a side facing the second exhaust gas introduction cell across a cellwall among the sides forming the cross sectional shape of the firstexhaust gas introduction cell is parallel to a side facing the firstexhaust gas introduction cell across the cell wall among the sidesforming the cross sectional shape of the second exhaust gas introductioncell, and distances between the parallel sides are the same.

In the honeycomb filter of the embodiments of the present invention, inthe cross section perpendicular to the longitudinal direction of theaforementioned cells, preferably the vertex portions of the polygonalcells are formed by curved lines.

The honeycomb filter having cells with vertex portions formed by curvedlines according to the above structure are not susceptible toconcentration of stress derived from heat or the like in the cornerportions of the cells. Thus, cracks hardly occur.

In the honeycomb filter according to the embodiments of the presentinvention, in the direction perpendicular to the longitudinal directionof the cells,

the exhaust gas emission cells, the first exhaust gas introductioncells, and the second exhaust gas introduction cells are preferablypoint-symmetrical polygons each having not more than eight sides.

The cells each having a point-symmetrical polygonal shape with not morethan eight sides can reduce the resistance caused upon flowing ofexhaust gas through the cells, and thus can further reduce the pressureloss.

In the honeycomb filter according to the embodiments of the presentinvention, preferably, the exhaust gas introduction cells include firstexhaust gas introduction cells and second exhaust gas introduction cellseach having a larger cross sectional area than each first exhaust gasintroduction cell in a direction perpendicular to the longitudinaldirection of the cells, each exhaust gas emission cell has an equal orlarger cross sectional area than each second exhaust gas introductioncell in a direction perpendicular to the longitudinal direction of thecells, the exhaust gas emission cells and the exhaust gas introductioncells are each in a shape formed by curved lines in the directionperpendicular to the longitudinal direction of the cells, and thethickness of the cell walls separating the first exhaust gasintroduction cells and the respective exhaust gas emission cells issmaller than the thickness of the cell walls separating the secondexhaust gas introduction cells and the respective exhaust gas emissioncells.

In the honeycomb filter according to the embodiments of the presentinvention, in a case where the thickness of the cell walls separatingthe first exhaust gas introduction cells and the respective exhaust gasemission cells is smaller than the thickness of the cell wallsseparating the second exhaust gas introduction cells and the respectiveexhaust gas emission cells exhaust gas easily passes through the cellwalls separating the first exhaust gas introduction cells and therespective exhaust gas emission cells at an early stage. Afteraccumulation of a certain amount of PMs, exhaust gas is likely to passthrough the cell walls separating the second exhaust gas introductioncells and the respective exhaust gas emission cells. The second exhaustgas introduction cells have a larger cross sectional area than the firstexhaust gas introduction cells, and the exhaust gas emission cells havean equal or larger cross sectional area than the respective secondexhaust gas introduction cells. For this reason, the above effects ofthe embodiments of the present invention are exerted.

In the honeycomb filter according to the embodiments of the presentinvention, in the direction perpendicular to the longitudinal directionof the cells,

preferably the exhaust gas introduction cells end the exhaust gasemission cells are each in a shape formed by curved lines, and

the thickness of the cell walls separating the first exhaust gasintroduction cells and the respective exhaust gas emission cells is 40to 75% the thickness of the cell walls separating the second exhaust gasintroduction cells and the exhaust gas emission cells.

In the honeycomb filter of the embodiments of the present invention, ifthe thickness of the cell walls separating the first exhaust gasintroduction cells and the exhaust gas emission cells is 40 to 75% thethickness of the cell walls separating the second exhaust gasintroduction cells and the exhaust gas emission cells, exhaust gaseasily passes through the cell walls separating the first exhaust gasintroduction cells and the exhaust gas emission cells at an initialstage. Then, after accumulation of a certain amount of PMs, exhaust gaspasses through the cell walls separating the second exhaust gasintroduction cells end the exhaust gas emission cells. Moreover, thesecond exhaust gas introduction cells each have a larger cross sectionalarea than each first exhaust gas introduction cell, and each exhaust gasemission cell has an equal or larger cross sectional area than eachsecond exhaust gas introduction cell. Thus, the aforementioned effectsof the embodiments of the present invention can be exerted.

If the thickness of the cell walls separating the first exhaust gasintroduction cells and the exhaust gas emission cells is less than 40%the thickness of the cell walls separating the second exhaust gasintroduction cells and the exhaust gas emission cells, the cell wallsseparating the first exhaust gas introduction cells and the exhaust gasemission cells need to be extremely thin. Consequently, the honeycombfilter has low mechanical strength. If the thickness of the cell wallsseparating the first exhaust gas introduction cells and the exhaust gasemission cells is more than 75% the thickness of the cell wallsseparating the second exhaust gas introduction cells and the exhaust gasemission cells, the former cells and latter cells do not have a bigdifference in the thickness. Consequently, the aforementioned pressureloss reduction effect may not be obtained.

In the honeycomb filter according to the embodiments of the presentinvention, in the direction perpendicular to the longitudinal directionof the cells,

the exhaust gas emission cells and the second exhaust gas introductioncells each preferably have a convex square cross section formed by fouroutwardly curved lines, whereas the first exhaust gas introduction cellseach preferably have a concave square cross section formed by fourinwardly curved lines.

The term “convex square” herein refers to a shape formed by fouroutwardly curved lines in the same length. The shape seems to be asquare having sides expanding outwardly from the geometrical center ofgravity. The term “concave square” herein refers to a shape formed byfour inwardly curved lines in the same length. The shape seems to be asquare having sides bending toward the geometrical center of gravity.

The exhaust gas emission cells, the first exhaust gas introductioncells, and the second exhaust gas introduction cells in this honeycombfilter having this structure have the aforementioned structures. Thus,the exhaust gas emission cells each have a larger cross sectional areathan each first exhaust gas introduction cells, and thereby providing ahoneycomb filter having the relations concerning the sizes of theexhaust gas emission cells, first exhaust gas introduction cells, andsecond exhaust gas introduction cells according to the embodiments ofthe present invention. Consequently, the effects of the embodiments ofthe present invention can be exerted.

In the honeycomb filter according to the embodiments of the presentinvention, in the direction perpendicular to the longitudinal directionof the cells, preferably the cross sectional area of each second exhaustgas introduction cell is equal in size to the cross sectional area ofeach exhaust gas emission cell, and

the cross sectional area of each first exhaust gas introduction cell is20 to 50% the size of the cross sectional area of each second exhaustgas introduction cell.

The honeycomb filter having the above structure can provide differencebetween the resistance caused upon flowing of exhaust gas through thefirst exhaust gas introduction cells and the resistance caused uponflowing of exhaust gas through the second exhaust gas introductioncells, thereby enabling effective suppression of the pressure loss.

If the cross sectional area of the first exhaust gas introduction cellsis less than 20% the size of the cross sectional area of the secondexhaust gas introduction cells, the cross sectional area of the firstexhaust gas introduction cells is too small. Consequently, highflow-through resistance occurs upon flowing of exhaust gas through thefirst exhaust gas introduction cells so that the pressure loss tends tobe high. If the cross sectional area of the first exhaust gasintroduction cells is more than 50% the size of the cross sectional areaof the second exhaust gas introduction cells, the difference inflow-through resistance between the first exhaust gas introduction cellsand the second exhaust gas introduction cells is small. Thus, thepressure loss is hardly reduced.

In the honeycomb filter of the embodiments of the present invention, theexhaust gas introduction cells preferably consist only of the firstexhaust gas introduction cells and the second exhaust gas introductioncells each having a larger cross sectional area than each first exhaustgas introduction cell in a direction perpendicular to the longitudinaldirection of the cells.

This is because a smaller number of first exhaust gas introduction cellseach having a smaller cross sectional area than each second exhaust gasintroduction cell provides a larger effective introduction cell area,thereby allowing PMs to be thinly and widely accumulated.

The honeycomb filter of the embodiments of the present inventionpreferably includes a plurality of honeycomb fired bodies combined withone another by adhesive layers residing therebetween, the honeycombfired bodies each having the exhaust gas emission cells, the firstexhaust gas introduction cells, and the second exhaust gas introductioncells, and each having an outer wall on the periphery thereof.

With regard to the structure formed by combining a plurality ofhoneycomb fired bodies to one another with adhesive layers residingtherebetween, since the cells forming one honeycomb fired body have thestructures according to the embodiments of the present invention, anaggregate of the honeycomb fired bodies can exert the effects of theembodiments of the present invention.

Moreover, the honeycomb fired bodies each having a smaller volumeenables to reduce the thermal stress that is generated upon productionand usage thereof, preventing damage such as cracks.

In the honeycomb filter of the embodiments of the present invention, theouter wall of the honeycomb fired body and the exhaust gas introductioncell adjacent to the outer wall have the following three patterns ofshapes:

(1) as illustrated in FIGS. 11B and 13A, the thickness at the outer wallis not uniform, and the first exhaust gas introduction cell and theexhaust gas emission cell adjacent to the outer wall have the sameshapes as the first exhaust gas introduction cell and the exhaust gasemission cell not adjacent to the outer wall, respectively;

(2) as illustrated in FIGS. 2A and 13B, the thickness of the outer wallis uniform, the first exhaust gas introduction cell adjacent to theouter wall has the same shape as the first exhaust gas introduction cellnot adjacent to the outer wall, and the exhaust gas emission celladjacent to the outer wall has a shape partially deformed, compared tothe cross sectional shape of the exhaust gas emission cell not adjacentto the outer wall, in accordance with the line along the inner wall,which forms the outer well, in the exhaust gas emission cell adjacent tothe outer wall; and

(3) as illustrated in FIGS. 12B and 13C, the thickness of the outer wallis uniform in accordance with the cross sectional shapes of the firstexhaust gas introduction cell and the exhaust gas emission cell adjacentto the outer wall, and the first exhaust gas introduction cell and theexhaust gas emission cell adjacent to the outer wall have the sameshapes as the first exhaust gas introduction cell and the exhaust gasemission cell not adjacent to the outer wall, respectively, which meansthat the outer wall is bending in accordance with the cross sectionalshapes of the first exhaust gas introduction cell and the exhaust gasemission cell adjacent to the outer wall.

In the honeycomb filter of the embodiments of the present invention,preferably, the outer wall has corner portions and in the exhaust gasintroduction cells and the exhaust gas emission cells adjacent to theouter wall, a side, which contacts the outer wall, is straight andparallel to a side corresponding to an outer periphery of the outer wallin a manner that the thickness of the outer wall except for the cornerportions is substantially uniform.

The pattern (2) in the above description of three patterns applies tothe honeycomb filter of such an embodiment.

The aforementioned structure enables to enhance the strength of thehoneycomb fired body by the outer wall but also suppress the partialvariation of the volume ratio of the exhaust emission cells and theexhaust gas introduction cells in the honeycomb fired body. As a result,the flow of exhaust gas becomes more uniform, lowering the pressureloss.

In the honeycomb filter of the embodiments of the present invention, thethickness of the cell wall of the honeycomb filter is preferably 0.10 to0.46 mm.

The cell wall having such a thickness is thick enough to capture PMs inthe gas and effectively suppresses an increase of the pressure loss. Asa result, the honeycomb filter of the embodiments of the presentinvention can sufficiently exert the effects of the embodiments of thepresent invention.

If the thickness is less than 0.10 mm, the cell wall is too thin, sothat the mechanical strength of the honeycombs filter is lowered. If thethickness is more than 0.46 mm, the cell wall is too thick, so that thepressure loss upon passage of exhaust gas through the cell wall becomesgreater.

In the honeycomb filter of the embodiments of the present invention, thecell walls preferably have a porosity of 40 to 65%.

If the cell walls have a porosity of 40 to 65%, the cell walls canfavorably capture PMs in exhaust gas. Also, the increase in the pressureloss derived from the cell walls can be suppressed. Thus, it is possibleto provide a honeycomb filter which has a low initial pressure loss andtends not to suffer an increase in the pressure loss even afteraccumulation of PMs.

In the case of the cell walls having a porosity of less than 40%,exhaust gas tends not to easily pas through the cell walls due to thesmall proportion of the pores in the cell walls, thereby leading to highpressure loss caused upon passage of exhaust gas through the cell walls.In the case of the cell walls having a porosity of more than 65%, themechanical characteristics of the cell walls are low. Consequently,cracks tend to occur during regeneration or the like.

In the honeycomb filter of the embodiments of the present invention, thecell walls preferably have pores having an average pore diameter of 8 to25μm.

If the average pore diameter of pores in the cell walls is less than 8μm, the pressure loss is high. If the average pore diameter of pores inthe cell walls is more than 25 μm, too large pores decrease the PMcapturing efficiency. Te pore diameter and the porosity are measured bya mercury injection method under conditions of the contact angle of 130degrees and the surface tension of 485 mN/m.

The honeycomb filter of the embodiments of the present inventionpreferably has a peripheral coat layer formed on the periphery thereof.

The periphery coat layer functions to protect cells inside thereof frommechanical damage. Thus, a honeycomb filter having excellent mechanicalcharacteristics such as compression strength is obtained.

In the honeycomb filter of the embodiments of the present invention, inthe cross section perpendicular to the longitudinal direction of thecells forming the honeycomb filter,

preferably the first exhaust gas introduction cells, the second exhaustgas introduction cells, and the exhaust gas emission cells each have auniform cross sectional shape except for the plugged portion in adirection perpendicular to the longitudinal direction of the cellsthoroughly from the end at the exhaust gas introduction side to the endat the exhaust gas emission side, the cross sectional shape of the firstexhaust gas introduction cells is different from the cross sectionalshape of the second exhaust gas introduction cells, and the crosssectional shape of the exhaust gas emission cells is different from thecross sectional shape of the first exhaust gas introduction cells. Here,“different” means “not congruent,” and but encompasses “similar.” Inother words, if the cross sectional shapes are similar to each other,the cross sectional shapes are considered different from each other.

Each first exhaust gas introduction cell itself has a uniform crosssectional shape at any cross section thereof, each second exhaust gasintroduction cell itself has a uniform cross sectional shape at anycross section thereof, and each exhaust gas emission cell has a uniformcross sectional shape at any cross section thereof. Each first exhaustgas introduction cell has a different cross sectional shape from eachsecond exhaust gas introduction cell. Each exhaust gas emission cell hasa different cross sectional shape from each first exhaust gasintroduction cell.

The honeycomb filter of the embodiments of the present inventionpreferably has the following structure. In the cross sectionperpendicular to the longitudinal direction of the cells, a cell unithaving a cell structure described below is two-dimensionally repeated,where the first exhaust gas introduction cells and the second exhaustgas introduction cells surrounding each exhaust gas emission cell in theunit are shared between adjacent cell units.

cell structure: each exhaust gas emission cell is adjacently surroundedfully by the exhaust gas introduction cells across the porous cellwalls, the exhaust gas introduction cells including first exhaust gasintroduction cells and second exhaust gas introduction cells each havinga larger cross sectional area than each first exhaust gas introductioncell in a direction perpendicular to the longitudinal direction of thecells, and each exhaust gas emission cell has an equal or larger crosssectional area than each second exhaust gas introduction cell in adirection perpendicular to the longitudinal direction of the cells, and

the exhaust gas introduction cells and the exhaust gas emission cellshave the following feature A or B in the cross section perpendicular tothe longitudinal direction of the cells:

A: the exhaust gas introduction cells and the exhaust gas emission cellsare each polygonal, and

a side forming the cross sectional shape of each first exhaust gasintroduction cell faces one of the exhaust gas emission cells, a sideforming the cross sectional shape of each second exhaust gasintroduction cell faces one of the exhaust gas emission cells, and theside of the first exhaust gas introduction cell is longer than the sideof the second exhaust gas introduction cell, or

a side forming the cross sectional shape of each first exhaust gasintroduction cell faces one of the exhaust gas emission cells, and noneof the sides forming the cross sectional shape of each second exhaustgas introduction cell faces the exhaust gas emission cells;

B: the exhaust gas introduction cells and the exhaust gas emission cellsare each in a shape formed by a curved line, and

the thickness of the cell walls separating the first exhaust gasintroduction cells and the exhaust gas emission cells is smaller thanthe thickness of the cell walls separating the second exhaust gasintroduction cells and the exhaust gas emission cells.

Such a honeycomb filter is preferable because the two-dimensionalrepetition of the cell unit forms a filter having a large capacity. Thefilter has an outer wall, and the cell units naturally do not spreadoutside the outer wall. The cell units are cut out properly to be fitinto the shape of the outer wall.

The embodiments will now be described with reference to the accompanyingdrawings, wherein like reference numerals designate corresponding oridentical elements throughout the various drawings.

The present invention is net limited to those embodiments, and may bemodified within a scope not changing the gist of the present invention.

First Embodiment

The following will discuss a honeycomb filter according to the firstembodiment of the present invention.

The honeycomb filter according to the first embodiment of the presentinvention includes a plurality of honeycomb fixed bodies. Each honeycombfired body includes exhaust gas emission cells each having an open endat an exhaust gas emission side and a plugged end at an exhaust gasintroduction side, and exhaust gas introduction cells each having anopen end at the exhaust gas introduction side and a plugged end at theexhaust gas emission side, the exhaust gas introduction cells includingfirst exhaust gas introduction cells and second exhaust gas introductioncells, and has an outer wall on the periphery thereof. The honeycombfired bodies are combined with one another by adhesive layers residingtherebetween.

In terms of the cells other than the cells adjacent to the outer wall,the exhaust gas emission cells are each adjacently surrounded fully bythe first exhaust gas introduction cells and the second exhaust gasintroduction cells across the porous cell walls.

In the cross section perpendicular to the longitudinal direction of thecells, each second exhaust gas introduction cell has a larger crosssectional area than each first exhaust gas introduction cell, and eachexhaust gas emission cell has the same cross sectional area as eachsecond exhaust gas introduction cell.

In the cross section perpendicular to the longitudinal direction of thecells, the exhaust gas emission cells and the exhaust gas introductioncells are each polygonal, and a side facing one exhaust gas emissioncell among the sides forming the cross sectional shape of the firstexhaust gas introduction cell is longer than a side facing one exhaustgas emission cell among the sides forming the cross sectional shape ofthe second exhaust gas introduction cell.

The cells adjacent to the outer wall include the first exhaust gasintroduction cells and the exhaust gas emission cells arrangedalternately with the first exhaust gas introduction cells. Each exhaustgas emission cell has a larger cross sectional area than each firstexhaust gas introduction cell in a direction perpendicular to thelongitudinal direction of the cells.

Specifically, the arrangement of two first exhaust gas introductioncells, one second exhaust gas introduction cell, and one exhaust gasemission cell is two-dimensionally repeated. Here, since the crosssectional areas thereof have the above relationships, the pluggedportions of the exhaust gas introduction cells are arranged in verticaland horizontal lines in the end face at the exhaust gas emission side.

The outer wall has corner portions. In the exhaust gas introductioncells and the exhaust gas emission cells adjacent to the outer wall, aside, which contacts the outer wall, is straight and parallel to a sidecorresponding to an outer periphery of the outer wall in a manner thatthe thickness of the outer wall except for the corner portions issubstantially uniform, and the exhaust gas emission cells adjacent tothe outer wall each have a shape partially deformed.

The exhaust gas introduction cells and the exhaust gas emission cellseach have a uniform cross sectional shape except for the pluggedportion, in a direction perpendicular to the longitudinal direction ofthe cells thoroughly from the end at the exhaust gas introduction sideto the end at the exhaust gas emission side.

The honeycomb fired body forming the honeycomb filter according to thefirst embodiment of the present invention includes silicon carbidegrains, and the grains include a silicon-containing oxide layer having athickness of 0.1 to 2 μm on the surface thereof. The aperture ratio ofthe end face at the exhaust gas emission side is not less than 20%.

FIG. 1 is a perspective view schematically illustrating one example ofthe honeycomb filter according to the first embodiment of the presentinvention.

FIG. 2A is a perspective view schematically illustrating one example ofthe honeycomb fired body forming the honeycomb filter shown in FIG. 1.FIG. 2B is an A-A line cross sectional view of the honeycomb fired bodyshown in FIG. 2A. FIG. 2C is a view of an end face at the exhaust gasemission side of the honeycomb fixed body in FIG. 2A.

The honeycomb filter 20 shown in FIG. 1 includes a ceramic block 18formed by combining a plurality of honeycomb fired bodies 10 with anadhesive layer 15 residing therebetween, and has a periphery coat layer16 for preventing leakage of emission gas on the periphery of theceramic block 18. The periphery coat layer 16 may be optionally formed.

Such a honeycomb filter including combined honeycomb fired bodies isalso referred to as an aggregated honeycomb filter.

The honeycomb fired body 10 has a rectangular pillar shape, and isroundly cornered at the corner portions in end faces thereof as shown inFIG. 2A. This prevents thermal stress concentration at the cornerportions to thereby prevent occurrence of damages such as cracks. Thecorner portions each may be chamfered in a manner to have a shape formedby straight lines.

In the honeycomb filter 20 according to the first embodiment, theexhaust gas emission cells each have an open end at the exhaust gasemission side and a plugged end at the exhaust gas introduction side,and the exhaust gas introduction cells each have an open end at theexhaust gas introduction side and a plugged end at the exhaust gasemission side. The plug preferably has high thermal conductivity, andspecifically, its thermal conductivity is preferably not less than 5W/mK.

In the honeycomb fired body 10 shown in FIG. 2A, FIG. 2B, and FIG. 2C,the exhaust gas emission cells 11 having an octagonal cross section areeach adjacently surrounded fully by the first exhaust gas introductioncells 12 each having a square cross section and the second exhaust gasintroduction cells 14 each having an octagonal cross section acrossporous cell walls therebetween. The first exhaust gas introduction cells12 and the second exhaust gas introduction cells 14 are alternatelyarranged around each exhaust gas emission cell 11. Each second exhaustgas introduction cell 14 has a larger cross sectional area than eachfirst exhaust gas introduction cell 12, and each exhaust gas emissioncell 11 has the same cross sectional area as each second exhaust gasintroduction cell 14. An outer wall 17 is formed on the periphery of thehoneycomb fired body 10. Cells adjacent to the outer wall 17 includeexhaust gas emission cells 11A and 11B and first exhaust gasintroduction cells 12A.

Each second exhaust gas introduction cell 14 and exhaust gas emissioncell 11 have octagonal cross sections, and the cross sections arecongruent with each other.

In the exhaust gas emission cells 11A and 11B and the exhaust gasintroduction cells 12A adjacent to the outer wall 17 in the honeycombfilter 20 according to the present embodiment, a side, which contactsthe outer wall 17, is straight and parallel to a side corresponding toan outer periphery of the outer wall 17 in a manner that the thicknessof the outer wall 17 is uniform except for the corner portions in thecross section perpendicular to the longitudinal direction of the cells.

The cross sections of the exhaust gas emission cells 11A adjacent to theouter wall 17 are partially cut so that the cross sectional shapes arechanged from octagons to hexagons. The cross sectional shape of thefirst exhaust gas introduction cells 12A may be in a partially cut shapebut is preferably congruent with the cross sectional shape of the firstexhaust gas introduction cells 12.

The cross sections of second exhaust gas introduction cells 11B at thecorner portions of the honeycomb fired body 10 have been changed fromoctagon to substantial pentagon including chamfered portions 110B formedby curved lines. The chamfered portions 110B of the exhaust gas emissioncells 11B illustrated in FIG. 2A are chamfered to be curved but may belinearly chamfered.

The aforementioned structure enables to not only enhance the strength ofthe honeycomb fired body by the outer wall but also further reduce thepartial variation in the volume ratio between the exhaust gas emissioncells and the exhaust gas introduction cells in the honeycomb firedbody. Consequently, the uniform flow of exhaust gas is improved. In thevicinity of the outer wall, exhaust gas smoothly flows into the firstexhaust gas introduction cells 12 and the cell wall and outer walleffectively function as a filter, so that the pressure loss is reduced.

In the honeycomb fired body 10, the exhaust gas emission cells 11A and11B and the exhaust gas introduction cells 2A are alternately arrangedalong the outer wall 17, and the exhaust as emission cells 11 is orderlyarranged inside the cells along the outer wall 17 with the first exhaustgas introduction cells 12 and the second exhaust gas introduction cellsresiding therebetween. As above, the exhaust gas emission cells 11, thefirst exhaust gas introduction cells 12, and the second exhaust gasintroduction cells 14 are arranged in a significantly ordered manner.

Each exhaust gas emission cell 11 and each second exhaust gasintroduction cell 14 have the same octagonal shape. The octagon ispoint-symmetry with respect to the center of gravity. In the octagon,all hypotenuse sides (14 a in FIGS. 6A to 6C) have the same length, andall vertical or horizontal sides (14 b in FIGS. 6A to 6C) have the samelength. Moreover, four first sides (the hypotenuse sides) and foursecond sides (the vertical or horizontal sides) are alternatelyarranged. The first sides and the second sides form an angle of 135°.

The term “hypotenuse side” generally refers to the longest side that isopposite to the right angle in a right-angled triangle. However, forconvenience for explanation, a “hypotenuse side” herein refers to a sidesuch as the side 14 a and the side 11 b that has a certain angle, exceptfor 90° and 0°, to below-mentioned four hypothetical segments. Fordifferentiation from this term, the term “vertical or horizontal side”herein refers to a side such as the side 14 b. and the side 11 a that isparallel to or vertical to the below-mentioned four hypotheticalsegments.

The hypothetical segments mentioned in the explanation of the terms“hypotenuse side” and “vertical or horizontal sides” refer to fourhypothetical segments (the four segments form a square) that do notcross the cross sectional figure of the exhaust gas emission cell 11,among hypothetical segments that connect geometric centers of gravity ofthe respective cross sectional figures of the four second exhaust gasintroduction cells arranged around the exhaust gas emission cell 11.

Each first exhaust gas introduction cell 12 has a square cross section.

In the cross section of the three kinds of cells which are adjacent toeach other, namely, the exhaust gas emission cell 11, the second exhaustgas introduction cell 14, and the first exhaust gas introduction cell12, the side 11 a facing the first exhaust gas introduction cell 12across the cell wall 13 among the sides of the octagonal exhaust gasemission cell 11 is parallel to the side 12 a facing the exhaust gasemission cell 11 across the cell wall 13 among the sides of the squarefirst exhaust gas introduction cell 12.

Also, the side 11 b facing the octagonal second exhaust gas introductioncell 14 across the cell wall 13 among the sides of the octagonal exhaustgas emission cells 11 is parallel to the side 14 a facing the exhaustgas emission cell 11 across the cell wall 13 among the sides of theoctagonal second exhaust gas introduction cell 14. Moreover, the side 12b facing the second exhaust gas introduction cell 14 across the cellwall 13 among the sides of the first exhaust gas introduction cell 12 isparallel to the side 14 b facing the first exhaust gas introduction cell12 across the cell wall 13 among the sides of the second exhaust gasintroduction cell 14. Furthermore, the distances between the parallelsides of all the above pairs are the same (see FIGS. 6A to 6C). That is,the distance between the parallel sides 11 a and 12 a, the distancebetween the parallel sides 11 b and 14 a, and the distance between theparallel sides 12 b and 14 b are the same.

Additionally, the exhaust gas emission cells 11, the first exhaust gasintroduction cells 12, and the second exhaust gas introduction cells 14are arranged in a manner satisfying the conditions below.

Among the hypothetical segments connecting the geometric centers ofgravity of the octagonal shapes of the four second exhaust gasintroduction cells 14 surrounding the exhaust gas emission cell 11, anintersection of the two segments crossing the octagonal shape region ofthe exhaust gas emission cell 11 is identical with the geometric centerof gravity of the octagonal cross section of the exhaust gas emissioncell 11.

Moreover, among the hypothetical segments connecting the geometriccenters of gravity of the octagonal shapes of the four second exhaustgas introduction cells 14, the four segments not crossing the octagonalshape region of the exhaust gas emission cell 11 forms a square, andmidpoints of the respective sides of the square are identical with thegeometric centers of gravity of the respective square shapes of the fourfirst exhaust gas introduction cells 12 surrounding the exhaust gasemission cell 11.

As described above, the octagonal exhaust gas emission cell 11 isadjacently surrounded by alternately arranged four pieces of the firstsquare exhaust gas introduction cells 12 and four pieces of the secondoctagonal exhaust gas introduction cells 14 across the cell walls 13 toform a single unit. The unit is two-dimensionally repeated, where thefirst exhaust gas introduction cells 12 and the second exhaust gasintroduction cells 14 in the unit are shared between adjacent cellunits, to form a honeycomb filter. Since the units share the firstexhaust gas introduction cells 12 and the second exhaust gasintroduction cells 14, the first exhaust gas introduction cell 12 andthe second exhaust gas introduction cell 14, that adjoin the exhaust gasemission cell 11 across the cell walls 13, also adjoin the exhaust gasemission cell 11 in the adjacent unit across the cell wall 13.

FIG. 7 is an enlarged cross sectional view perpendicular to thelongitudinal direction of the honeycomb filter. FIG. 7 illustrates howeach cell unit (cell structure) is two-dimensionally, i.e. in X and Ydirections shown in FIG. 7, repeated in the case where the secondexhaust gas introduction cells 14 and the exhaust gas emission cells 11are octagonal, the first exhaust gas introduction cells 12 are square inthe cross section of the cells, and the aforementioned conditions aresatisfied, and also illustrates how the first exhaust gas introductioncells 12 and the second exhaust gas introduction cells 14 are sharedbetween the cell units (cell structure).

A cell unit 1, a cell unit 2, and a cell unit 3 each have a structure inwhich the exhaust gas emission cell 11 is fully surrounded byalternately arranged four pieces of the first exhaust gas introductioncells 12 and four pieces of the second exhaust gas introduction cells 14across the cell walls 13 in a manner satisfying the aforementionedconditions. The cell unit 2 has the same structure as that of the cellunit 1. The cell unit 2 is adjacent to the cell unit 1 in the Xdirection while sharing one piece of the first exhaust gas introductioncell 12 and two pieces of the second exhaust gas introduction cells 14with the cell unit 1. The cells shared between the cell unit 1 and thecall unit 2 are depicted as “shared portion 2” in FIG. 7. The cell unit3 has the same structure as that of the cell unit 1. The cell unit 3 isadjacent to the cell unit 1 in the Y direction while sharing one pieceof the first exhaust gas introduction cell 12 and two pieces of thesecond exhaust gas introduction cells 12 with the cell unit 1. The cellsshared between the cell unit 1 and the cell unit 3 are depicted as“shared portion 1” in FIG. 7.

Meanwhile, FIG. 7 shows four segments H, I, J, and K that do not crossthe octagonal shape region of the exhaust gas emission cell 11, andhypothetical two segments L and M that cross the octagonal shape regionof the exhaust gas emission cell 11, among hypothetical segmentsconnecting the geometric centers of gravity of the respective octagonalshapes of the four pieces of the second exhaust gas introduction cells14. The “shared portion 2” is depicted by cross-hatching with segmentsin the same direction as that of the segment M, and the “shared portion1” is depicted by cross-hatching with segments in the same direction asthat of the segment L.

As shown in FIG. 7, an intersection of the two segments L and M isidentical with the geometric center of gravity of the exhaust gasemission cell 11.

With regard to the cell shapes in the honeycomb filter 20 shown in FIGS.1, 2A to 2C, and 6A to 6C, the exhaust gas emission cells 11 and thesecond exhaust gas introduction cells 14, except for the exhaust gasemission cells 11A and 11B adjacent to the outer wall 17, each have anoctagonal cross section, and the first exhaust gas introduction cells 12and 12A each have a square cross section. However, the cross sectionalshapes of the exhaust gas emission cells and the exhaust gasintroduction cells of the present invention are not limited to the aboveshapes, and may be all square as mentioned below, or may be combinationsof other polygons.

Moreover, the exhaust gas emission cells 11, the first exhaust gasintroduction cells 12, and the second exhaust gas introduction cells 14,which are polygons in the cross section thereof, may be roundly corneredso that the vertex portions are formed by curved lines in the crosssection.

Examples of the curved lines include curved lines obtained by dividing acircle into quarters, and curved lines obtained by dividing an ellipseinto four equal parts linearly along the long axis and the axisperpendicular to the long axis. In particular, the vertex portions ofthe cells having a rectangular cross section are preferably formed bycurved lines in the cross section. This prevents stress concentration atthe vertex portions, thereby preventing cracks in the cell walls.

Furthermore, the honeycomb filter 20 may partially include cells formedby curved lines such as a cell having a circle cross section.

The following description is not applicable to the exhaust gas emissioncells 11A and 11B adjacent to the outer wall 17.

The cross sectional area of each first exhaust gas introduction cell 12is preferably 20 to 50%, and more preferably 22 to 45% the site of thecross sectional area of each second exhaust gas introduction cell 14.

In the honeycomb filter 20 shown in FIGS. 1, 2A to 2C, and 6A to 6C, thecross sectional area of each exhaust gas emission cell 11 is equal tothe cross sectional area of each second exhaust gas introduction cell14; however, the cross sectional area of each exhaust gas emission cell11 may be larger than the cross sectional area of each second exhaustgas introduction cell 14.

The cross sectional area of each exhaust gas emission cell 11 ispreferably 1.05 to 1.5 times the size of the cross sectional area ofeach second exhaust gas introduction cell 14.

Moreover, the side 12 a facing the exhaust gas emission cell 11 amongthe sides forming the cross sectional shape of the first exhaust gasintroduction cell 12 is longer than the side 14 a facing the exhaust gasemission cell 11 among the sides forming the cross sectional shape ofthe second exhaust gas introduction cell 14. The sides 12 a and 14 a aresides facing the exhaust gas emission cell 11 according to theaforementioned definition of the embodiments of the present invention.

The ratio of the length of the side 14 a of the second exhaust gasintroduction cell 14 to the length of the side 12 a of the first exhaustgas introduction cell 12 (length of the side 14 a/length of the side 12a) is not particularly limited, and is preferably not more than 0.8,more preferably not more than 0.7, and still more preferably not morethan 0.5.

As shown in FIG. 2B, exhaust gas G₁ (exhaust gas is represented by anarrow G₁ which shows the flow of exhaust gas in FIG. 2B) having flowedinto the first exhaust gas introduction cells 12 or the second exhaustgas introduction cells 14 inevitably passes through the cell walls 13which separates the exhaust gas emission cells 11 and the respectivefirst exhaust gas introduction cells 12 or the respective second exhaustgas introduction cells 14, and then flows out of the exhaust gasemission cells 11. Upon passage of exhaust gas G₁ through the cell walls13, PMs and the like in the exhaust gas are captured so that the cellwalls 13 function as filters.

The exhaust gas emission cells 11, the first exhaust gas introductioncells 12, and the second exhaust gas introduction cells 14 allow flow ofgas such as exhaust gas as described above. For flow of exhaust gas inthe direction shown in FIG. 2B, an end at a first end face 10 f of thehoneycomb fired body 10 (the end on the side at which the exhaust gasemission cells 11 are plugged) is referred to as an exhaust gasintroduction side end, and an end at a second end face 10 r of thehoneycomb fired body 10 (the end on the side at which the first exhaustgas introduction cells 12, and the second exhaust gas introduction cells14 are plugged) is referred to as an exhaust gas emission side end. Thelatter end is shown in FIG. 2C.

The honeycomb filter 20 having the aforementioned structure not only hashigh soot mass limit and can reduce the initial pressure loss ascompared with conventional honeycomb filters but also can reduce therate of increase in the pressure loss even after accumulation of aconsiderable amount of PMs on the cell walls as explained in the effectsof the honeycomb filter according to the embodiments of the presentinvention. The honeycomb filter 20 can significantly reduce the pressureloss throughout the use from the initial stage to after accumulation ofPMs in close to the limit amount.

In the honeycomb filter 20 having the structure illustrated in FIGS. 2Ato 2C, since the thickness of the outer wall 17 is uniform over thewhole length, the strength of the honeycomb fired body is enhanced bythe outer wall and the partial variation in the volume ratio is furtherreduced between the exhaust gas emission cells and the exhaust gasintroduction cells in the honeycomb fired body. Consequently, theuniform flow of exhaust gas is improved so that the pressure loss can bereduced.

In terms of the cells adjacent to the outer wall 17, since the exhaustgas emission cells 11A and 11B are each adjacent to the exhaust gasintroduction cell 12A, exhaust gas can pass through the inside of thecuter wall 17, which means that pert of the outer wall 17 can be used asa filter. As a result, the pressure loss is further reduced.

Moreover, since the outer wall 17 is chamfered, stress is prevented fromconcentrating on the corner portions of the honeycomb fired body 10, sothat cracks hardly occur in the corner portions of the honeycomb firedbody 10. When the cross sectional shape of the exhaust gas emission cell11B positioned at the corner portion is a pentagon formed by straightlines, not having the chamfered portion 110B formed by curved lines, theexhaust gas emission cell 11B positioned close to the portion of thehoneycomb fired body 10 is likely to have a stress concentrated thereonto easily have cracks. In the honeycomb filter 20, however, since theexhaust gas emission cells 11B each have the chamfered portion 110B,cracks hardly occurs therein.

The honeycomb filter 20 according to the first embodiment includes aplurality of honeycomb fired bodies 10. The material constituting thehoneycomb fired bodies 10 is silicon carbide grains, and the siliconcarbide grains are provided with a silicon-containing oxide layer(silica layer) on the surface thereof. The material includes grains witha silicon carbide content of not less than 60 wt %.

The thickness of the cell walls separating the cells is preferablyuniform at any part in the honeycomb fired body 10 forming the honeycombfilter 20 according to the first embodiment. The thickness of the cellwalls is preferably 0.10 to 0.46 mm, and more preferably 0.12 to 0.35mm. The thickness of the outer wall 17 is preferably 0.10 to 0.50 mm.Meanwhile, the thickness of the cell wall is a value measured as thethickness D shown in FIG. 3B based on the aforementioned definition.

The thickness of the plugged portion is preferably 1.0 to 5.0 mm.

The porosity of the cell walls and the outer wall is preferably 40 to65%, and more preferably 40 to 60% in the honeycomb fired body 10forming the honeycomb filter 20 according to the first embodiment.

The average pore diameter of pores in the cell walls is preferably 8 to25 μm in the honeycomb fired body 10 forming the honeycomb filter 20according to the first embodiment.

The number of the cells per unit area is preferably 31 to 62 pcs/cm²(200 to 400 pcs/inch²) in the cross section of the honeycomb fired body10.

The cell walls preferably constitute 20% or more of the cross section ofthe honeycomb fired body 10. If the proportion is less than 20%, thehoneycomb fired body may have insufficient strength.

The honeycomb filter 20 according to the embodiments of the presentinvention is formed by combining, with an adhesive layer residingtherebetween, a plurality of the honeycomb fired bodies each having anouter wall on the periphery thereof. An adhesive layer that combines thehoneycomb fired bodies is prepared by applying an adhesive paste thatcontains an inorganic binder and inorganic particles, and drying theadhesive paste. The adhesive layer may further contain at least one ofan inorganic fiber and a whisker.

The adhesive layer preferably has a thickness of 0.5 to 2.0 mm.

The honeycomb filter according to the first embodiment of the presentinvention may have a periphery coat layer on the periphery thereof. Thematerial of the periphery coat layer is preferably the same as thematerials of the adhesive layer.

The periphery coat layer preferably has a thickness of 0.1 to 3.0 mm.

The following will discuss a method of manufacturing the honeycombfilter according to the first embodiment of the present invention.

(1) A molding process for manufacturing a honeycomb molded body isperformed by extrusion molding a wet mixture containing silicon carbidepowder and a binder.

Specifically, silicon carbide powders having different average particlesizes, an organic binder, a pore-forming agent, a liquid plasticizer, alubricant, and water are mixed to prepare a wet mixture formanufacturing a honeycomb molded body.

Examples of the pore-forming agent include balloons that are fine hollowspheres including oxide-based ceramics, spherical acrylic particles,graphite, and starch.

The balloons are not particularly limited, and examples thereof includealumina balloon, glass micro balloon, shirasu balloon, fly ash balloon(FA balloon), and mullite balloon. Alumina balloon is preferable amongthese.

Then, the wet mixture is charged into an extrusion molding machine andextrusion-molded to manufacture honeycomb molded bodies in predeterminedshapes.

Here, a honeycomb molded body is manufactured with a die that can make across sectional shape having the cell structure (shapes and arrangementof the cells) shown in FIGS. 2A to 2C. The die has a design that allowsfox the production of a honeycomb filter in which plugged portions ofexhaust gas introduction cells are arranged in vertical and horizontallines with the cell walls residing therebetween in the end face at theexhaust gas emission side.

(2) The honeycomb molded bodies are cut at a predetermined length anddried with use of a drying apparatus such as a microwave dryingapparatus, a hot-air drying apparatus, a dielectric drying apparatus, areduced-pressure drying apparatus, a vacuum drying apparatus, or afreeze drying apparatus. Then, predetermined cells are plugged byfilling the cells with a plug material paste to be a plug material(plugging process).

Here, the wet mixture may be used as the plug material paste.

(3) Then, the honeycomb molded body is heated at 300° C. to 650° C. in adegreasing furnace to remove organic matters in the honeycomb moldedbody (degreasing process). The degreased honeycomb molded body istransferred to a firing furnace and fired at 2000° C. to 2200° C.(firing process), and then an oxidation process is further carried outin an oxygen atmosphere at 1100 to 1450° C. for 1 to 15 hours. In thismanner, the honeycomb fired body having the configuration as shown inFIGS. 2A to 2C is manufactured.

More preferably, the oxidation process is carried out at 1200 to 1400°C. for 3 to 10 hours although the conditions depend an the desiredthickness of the oxide layer (silica layer). The oxidation process maybe carried out after firing the honeycomb molded body or after thelater-described process for preparing a honeycomb filter, under theabove-mentioned conditions.

The plug material paste filled into the end of the cells is fired byheating to be a plug material.

Conditions for cutting, drying, plugging, degreasing, and firing may beconditions conventionally used for manufacturing honeycomb fired bodies.

(4) A plurality of the honeycomb fired bodies are stacked in series withthe adhesive paste residing therebetween on a support table to combinethe honeycomb fired bodies (combining process) so that a honeycombaggregate body including the plurality of stacked honeycomb fired bodiesis manufactured.

The adhesive paste contains, for example, an inorganic binder, anorganic hinder, and inorganic particles. The adhesive paste may furthercontain inorganic fibers and/or whisker.

Examples of the inorganic particles in the adhesive paste includecarbide particles, nitride particles, and the like. Specific examplesthereof include inorganic particles made from silicon carbide, siliconnitride, boron nitride, and the like. Each of these may be used alone,or two or more of these may be used in combination. Among the inorganicparticles, silicon carbide particles are preferable due to theirsuperior thermal conductivity.

Examples of the inorganic fibers and/or whisker in the adhesive pasteinclude inorganic fibers and/or whisker made from silica-alumina,mullite, alumina, and silica. Each of these may be used alone or two ormore of these may be used in combination. Alumina fibers are preferableamong the inorganic fibers. The inorganic fibers may be biosolublefibers.

Furthermore, balloons that are fine hollow spheres including oxide-basedceramics, spherical acrylic particles, graphite, or the like may beadded to the adhesive paste, if necessary. The balloons are notparticularly limited, and examples thereof include alumina balloon,glass micro balloon, shirasu balloon, fly ash balloon (FA balloon),mullite balloon, and the like.

(5) The honeycomb aggregate body is then heated to solidify the adhesivepaste, whereby a rectangular pillar-shaped ceramic block ismanufactured.

The heating and solidifying of the adhesive paste may be performed underconditions that have been conventionally employed for manufacturinghoneycomb filters.

(6) The ceramic block is subjected to cutting (cutting process).

Specifically, the periphery of the ceramic block is cut with a diamondcutter, whereby a ceramic block whose periphery is cut into asubstantially round pillar shape is manufactured.

(7) A peripheral coating material paste is applied to the peripheralface of the substantially round pillar-shaped ceramic block, and isdried and solidified to form a periphery coat layer (periphery coatlayer forming process).

The adhesive paste may be used as the peripheral coating material paste.Alternatively, the peripheral coating material paste may be a pastehaving a composition different from that of the adhesive paste.

The periphery coat layer is not necessarily formed and may be formed, ifnecessary.

The peripheral shape of the ceramic block is adjusted by the peripherycoat layer, and thereby a round pillar-shaped honeycomb filter isobtained.

The honeycomb filter including the honeycomb fired bodies can bemanufactured through the aforementioned processes.

Although the honeycomb filter having a predetermined shape ismanufactured by cutting, the honeycomb filter may also be allowed tohave a predetermined shape such as round-pillar shape as follows:honeycomb fired bodies of a plurality of shapes, each having an outerwall on the periphery thereof, are manufactured in the honeycomb firedbody manufacturing process, and then the honeycomb fired bodies of aplurality of shapes are combined with one another with the adhesivelayer residing therebetween. In this case, the cutting process can beomitted.

Hereinafter, the effects of the honeycomb filter according to the firstembodiment of the present invention are listed.

(1) The honeycomb filter according to the embodiment has improvedstrength attributable to the oxide layer. Additionally, the pluggedpotions, which are arranged in vertical and horizontal lines in the endface at the exhaust gas emission side of the honeycomb filter, improvethe thermal conductivity and exhibit heat releasing performance, therebyincreasing the soot mass limit of the honeycomb filter.

(2) The honeycomb filter according to the embodiment can not only reducethe initial pressure loss as compared with conventional honeycombfilters but also reduce the rate of increase in the pressure loss evenafter accumulation of a considerable amount of PMs on the cell walls.The honeycomb filter can significantly reduce the pressure lossthroughout the use from the initial stage to after accumulation of PMsin close to the limit amount.

(3) In the honeycomb filter according to the present embodiment, thecross sectional area of each first exhaust gas introduction cell may be20 to 50% the size of the cross sectional area of each second exhaustgas introduction cell.

This setting of the cross sectional area ratio of the first exhaust gasintroduction cell and the second exhaust gas introduction cell canprovide difference between the resistance caused upon flowing of exhaustgas through the first exhaust gas introduction cells and the resistancecaused upon flowing of exhaust gas through the second exhaust gasintroduction cells, thereby enabling effective reduction of the pressureloss.

(4) In the honeycomb filter according to the present embodiment, theratio of the length of the side of the second exhaust gas introductioncell facing the exhaust gas emission cell to the length of the side ofthe first exhaust gas introduction cell facing the exhaust gas emissioncell may be not more than 0.8.

The aforementioned ratio of the length of the side of the first exhaustgas introduction cell to the length of the side of the second exhaustgas introduction cell enables easier passage of exhaust gas through thecell walls separating the exhaust gas emission cells and the firstexhaust gas introduction cells, effective suppression of the initialpressure loss, and prevention of an increase in the rate of increase ofthe pressure loss after accumulation of PMs.

(5) In the honeycomb filter according to the present embodiment, thethickness of the cell walls separating the cells may be uniform at anypart.

This setting for the entire thickness of the cell walls enables toprovide a honeycomb filter having the same effects at any part thereof.

(6) In the honeycomb filter according to the present embodiment, thethickness of the cell walls may be 0.10 to 0.46 mm.

The cell walls having the aforementioned thickness are sufficient forcapturing PMs in exhaust gas, and also enable efficient suppression ofincrease in the pressure loss.

(7) In the honeycomb filter according to the present embodiment, theporosity of the cell walls and the outer wall forming the honeycombfilter may be 40 to 65%.

The cell walls having the aforementioned porosity can favorably capturePMs in exhaust gas. Also, the increase in the pressure loss derived fromthe cell walls can be suppressed.

(8) In the honeycomb filter according to the present embodiment, theaverage pore diameter of pores in the cell walls may be 8 to 25 μm inthe honeycomb fired body forming the honeycomb filter.

The above average pore diameter of pores in the cell walls enables tocapture PMs at a high capturing efficiency while suppressing an increasein the pressure loss.

(9) In the honeycomb filter according to the present embodiment, theexhaust gas introduction cells and the exhaust gas emission cells eachhave a uniform cross sectional shape except for the plugged portion in adirection perpendicular to the longitudinal direction of the cellsthoroughly from the end at the exhaust gas introduction side to the endat the exhaust gas emission side.

Thus, the entire honeycomb filter can exert the same or similar effectsso that disadvantages caused by local shape variations in the honeycombfilter can be prevented.

(10) In the honeycomb filter according to the embodiment, theaforementioned structure enables to not only enhance the strength of thehoneycomb fired body by the outer wall but also further reduce thepartial variation in the volume ratio between the exhaust gas emissioncells and the exhaust gas introduction cells in the honeycomb firedbody. Consequently, the uniform flow of exhaust gas is improved so thatthe pressure loss can be reduce.

Hereinafter, examples are given for more specifically describing thefirst embodiment of the present invention. However, the presentinvention is not limited only to the examples.

EXAMPLE 1

A mixture was obtained by mixing 56.3% by weight of a silicon carbidecoarse powder having an average particle size of 22 μm and 24.1% byweight of a silicon carbide fine powder having an average particle sizeof 0.5 μm. To the mixture were added 4.4% by weight of an organic bindermethylcellulose), 0.8% by weight of a lubricant (UNILUB, manufactured byNOF Corporation), 0.8% by weight of glycerin, 2.2% by weight of oleicacid, and 11.3% by weight of water and then kneaded to prepare a wetmixture. Thereafter, the wet mixture was extrusion-molded (moldingprocess).

This process provided a raw honeycomb molded body which had the sameshape as that of the honeycomb fixed body 10 shown in FIG. 2A and inwhich the cells were not plugged.

Next, the raw honeycomb molded body was dried using a microwave dryingapparatus to obtain dried honeycomb molded bodies. Then, predeterminedcells of the dried honeycomb molded body were plugged by filling thecells with a plug material paste.

Specifically, the cells are plugged in a manner that the end at theexhaust as introduction side and the end at the exhaust gas emissionside are plugged at the positions shown in FIG. 6A.

The wet mixture was used as the plug material paste. Thereafter, thedried honeycomb molded body, which has predetermined cells filled withthe plug material pasta, was dried with a drying apparatus again.

Subsequently, the dried honeycomb molded bodies after plugging of cellswere degreased at 400° C. (degrease treatment) and then fired at 2200°C. (firing treatment) under normal pressure argon atmosphere for threehours.

Subsequently, the oxidation process is carried out at 1200° C. in anoxygen atmosphere for 3 hours.

In this manner, a rectangular pillar-shaped honeycomb fired body wasmanufactured.

The measurement mentioned below of the length of sides and the crosssectional area can be performed by the aforementioned image analysis ofan electron microscope photograph and by use of the aforementioned grainsize distribution measurement software (Mac-View (Version 3.5), producedby Mountech Co. Ltd.). The thickness of the oxide layer can be measuredwith FIB-TEM.

The manufactured honeycomb fired body was the honeycomb fired body 10shown in FIGS. 2A and 2B formed of a silicon carbide sintered body thatincluded silicon carbide grains with a 0.2 μm thick oxide layer on thesurface thereof and had a porosity of 40.5%, an average pore diameter of10.5 μm, a size of 34.3 mm×34.3 μm×177.8 mm, the number of cells (celldensity) of 310 pcs/inch², a thickness of cell walls of 0.18 mm, and athickness of each plugged portion of 3 mm.

The exhaust gas emission cell 11 was adjacently surrounded fully by thefirst exhaust gas introduction cells 12 and 12A and the second exhaustgas introduction cells 14 in the cross section perpendicular to thelongitudinal direction of the manufactured honeycomb fired body 10. Thefirst exhaust gas introduction cells 12 and 12A had square crosssections, and the length of sides forming the cross sections of thefirst exhaust gas introduction cells 12 and 12A was 1.02 mm.

The second exhaust gas introduction cells 14 had octagonal crosssections. The length of the hypotenuse side of the second exhaust gasintroduction cells facing the exhaust gas emission cell 11 was 0.32 mm,and the vertical or horizontal sides not facing the exhaust gas emissioncell 11 were 1.13 mm.

In other words, the length of the side facing the exhaust gas emissioncell 11 among the sides forming the cross sectional shape of the secondexhaust gas introduction cell 14 was 0.28 times as long as the sidefacing the exhaust gas emission cell 11 among the sides forming thefirst exhaust gas introduction cell 12.

In the exhaust gas emission cells 11B at the four corners, the length ofsides adjacent to the outer wall 17 was 1.30 mm, the length of thevertical or horizontal sides was 1.08 mm, the length of the hypotenusesides was 0.32 mm, and the cross sectional area was 1.67 mm².

In the exhaust gas emission cells 11A, the length of the side adjacentto the outer wall 17 was 1.58 mm, the length of the vertical sideparallel to the side adjacent to the outer wall 17 was 1.13 mm, thelength of the horizontal side connected at a right angle to the sideadjacent to the outer wall 17 was 1.08 mm, the length of the hypotenuseside was 0.32 mm, and the cross sectional area was 2.00 mm².

The exhaust gas emission cell 11 had an octagonal cross section, and theshape of the cross section was the same as that of the second exhaustgas introduction cell 14. The length of the hypotenuse side facing thesecond exhaust gas introduction cell 14 was 0.32 mm, and the vertical orhorizontal sides facing the first exhaust gas introduction cells 12 was1.13 mm.

The thickness of the outer wall 17 was 0.35 mm.

The cross sectional area of the first exhaust gas introduction cell 12was 1.05 mm², and the cross sectional areas of the second exhaust gasintroduction cell 14 and the exhaust gas emission cell 11 were both 2.39mm². In other words, the cross sectional area of the first exhaust gasintroduction cell 12 was 44% the size of the cross sectional area of thesecond exhaust gas introduction cell 14.

Moreover, the cross sectional area of the exhaust gas emission cell 11was equal in size to the cross sectional area of the second exhaust gasintroduction cell 14, and was larger than the cross sectional area ofthe first exhaust gas introduction cell 12.

The honeycomb fired body was in a rectangular pillar shape that wasroundly cornered at the corner portions in end faces thereof.

Next, a plurality of the honeycomb fired bodies were combined using anadhesive paste containing 30% by weight of alumina fibers having anaverage fiber length of 20 μm, 21% by weight of silicon carbideparticles having an average particle size of 0.6 μm, 15% by weight ofsilica sol, 5.6% by weight of carboxymethyl cellulose, and 28.4% byweight of water. Subsequently. the adhesive layer was dried andsolidified at 120° C. to form an adhesive layer, and thereby apillar-shaped ceramic block was manufactured.

The periphery of the pillar-shaped ceramic block was cut out using adiamond cutter to manufacture a substantially round pillar-shapedceramic block.

Subsequently, a sealing material paste having the same composition asthat of the adhesive paste was applied to the peripheral face of theceramic block. The sealing material paste was dried and solidified at120° C. to form a periphery coat layer. In this manner a roundpillar-shaped honeycomb filter was manufactured.

The diameter of the honeycomb filter was 266.7 mm, and the length in thelongitudinal direction was 177.8 mm.

Comparative Example 1

A raw honeycomb molded body was obtained in the same molding process asin Example 1. Subsequently the raw honeycomb molded body was dried usinga microwave drying apparatus to manufacture a dried honeycomb moldedbody. Then, predetermined cells of the dried honeycomb molded body wereplugged by filling the cells with a plug material paste.

The positions of the cells to be plugged were changed from those inExample 1 as follows. The octagonal cells were all plugged in the endface corresponding to the end at the exhaust gas emission sides and thesquare cells were all plugged in the end face corresponding to the endat the exhaust gas introduction side so that the cells were alternatelyplugged in both of the end faces.

Consequently, a honeycomb molded body was obtained in which the end atthe exhaust gas introduction side and the end at the exhaust gasemission side were plugged at the positions shown in FIG. 21B.

A honeycomb fired body 130 shown in FIGS. 21A and 21B were manufacturedthrough the same processes as in Example 1 so that a honeycomb filter120 was manufactured.

In the cross section of the manufactured honeycomb fired body 130 in thedirection perpendicular to the longitudinal direction of the cells, allthe exhaust gas introduction cells 132 had an octagonal cross sectionexcept for the exhaust gas introduction cells 132A and 132B which wereadjacent to the outer wall 137.

The sides facing the exhaust gas emission cell 131 were vertical orhorizontal sides having a length of 1.33 mm.

The sides facing other exhaust gas introduction cells 132, 132A, and132B were hypotenuse sides having a length of 0.32 mm.

All the exhaust gas emission cells 131 and 131A had a square crosssection. The length of the sides forming the cross sections of theexhaust gas emission cells 131 and 131A was 1.02 mm.

In the exhaust gas introduction cells 132B at the four corners, thelength of sides adjacent to the outer wall 137 was 1.30 mm, the lengthof the vertical or horizontal sides was 1.08 mm, the length of thehypotenuse sides was 0.32 mm, and the cross sectional area was 1.67 mm².

In the exhaust gas introduction cells 132A, the length of the sideadjacent to the outer wall 137 was 1.58 mm, the length of the verticalside parallel to the side adjacent to the outer wall 17 was 1.13 mm, thelength of the horizontal line connected at a right angle to the sideadjacent to the outer wall 17 was 1.08 mm, the length of the hypotenuseside was 0.32 mm, and the cross sectional area was 2.00 mm².

The thickness of the cell walls 133 was 0.18 mm, and the thickness ofthe outer walls was 0.35 mm.

The cross sectional area of the exhaust gas introduction to cell 132 was2.39 mm², and the cross sectional area of the exhaust gas emission cell131 as 1.05 mm².

The honeycomb filters manufactured in Example 1 and Comparative Example1 were evaluated for the presence of cracks caused by thermal shock andmeasured for an initial pressure loss using an initial pressure lossmeasuring apparatus as shown in FIG. 8, and a relation between theamount of captured PMs and the pressure loss using a pressure lossmeasuring apparatus as shown in FIG. 9.

(Evaluation of the Presence of Cracks Caused by Thermal Impact)

A 34.3 mm×34.3 mm×171.8 mm sample piece of a honeycomb fired body wascut out from the honeycomb filter, and exhaust gas from a diesel enginewas allowed to pass through the honeycomb fired body sample from the endat the exhaust gas introduction side until 1.57 g of PM was accumulated.The honeycomb unit with accumulated PM was transferred to a furnace at700° C. in a nitrogen atmosphere. Air was allowed to pass through thehoneycomb fired body from the introduction side at 0.4 m/s for 10minutes while the above-mentioned conditions were maintained. Then, thehoneycomb fired body was taken out and visually observed for thepresence of cracks. No crack was observed in the honeycomb fired bodycut out from the honeycomb filter of Example 1, whereas the honeycombfired body cut out from the honeycomb filter of Comparative Example 1had visible cracks. These results confirmed that the soot mass limit ofthe honeycomb filter of Example 1 is high. Its high soot mass limit ispresumably attributable to the improved strength of the honeycomb firedbodies and the heat releasing performance of the plugged portions whichare arranged in vertical and horizontal lines with the cell wallsresiding therebetween in the end face at the exhaust gas emission side.

(Measurement of Initial Pressure Loss)

FIG. 8 is an explanatory diagram schematically illustrating an apparatusfor measuring the initial pressure loss.

An initial pressure loss measuring apparatus 210 includes a blower 211,an exhaust gas pipe 212 connected to the blower 211, a metal casing 213in which the honeycomb filter 20 is fixed, and a pressure gauge 214 inwhich pipes are arranged so that the pressure in front and back of thehoneycomb filter 20 can be measured. Specifically, in the initialpressure loss measuring apparatus 210, the pressure loss is measured byflowing gas through the honeycomb filter 20 and measuring the pressurein front and back of the honeycomb filter.

The blower 211 was operated three times to flow gas at a rate of 600m³/h, 800 m³/h, and 1200 m³/h. In each operation, the pressure loss infive minutes from the start was measured.

FIG. 10B is a graph showing a relation between the gas flow rate and theinitial pressure loss measured in Example 1 and Comparative Example 1.

The graph in FIG. 10B clearly shows that, in the honeycomb filteraccording to Comparative Example 1, the initial pressure loss was 0.59Pa, 0.92 Pa, and 1.76 Pa for the gas flow rate of 600 m³/h, 800 m³/h,and 1200 m³/h, respectively. In the honeycomb filter according toExample 1, the initial pressure loss was 0.49 Pa, 0.74 Pa, and 1.38 Pafor the gas flow rate of 600 m³/h, 800 m³/h, and 1200 m³/h,respectively, which was lower than that in Comparative Example 1. Inparticular, as the flow rate increases, the difference with ComparativeExample 1 becomes more significant.

FIG. 9 is an explanatory diagram schematically illustrating a method formeasuring the pressure loss.

The pressure loss measuring apparatus 310 has the following structure: ahoneycomb filter 20 fixed inside a metal casing 313 is disposed in anexhaust gas tube 312 of a 12.8-liter diesel engine 311, and a pressuregauge 314 is attached in a manner such that it can detect the pressurein front and back of the honeycomb filter 20. The honeycomb filter 20 isdisposed such that the end at the exhaust gas introduction side iscloser to the exhaust gas tube 312 of the diesel engine 311. Namely, thehoneycomb filter 20 is disposed to allow exhaust gas to flow in thecells which are open at the exhaust gas introduction side end.

The diesel engine 311 was operated with the number of rotation of 1800rpm and a torque of 2000 Nm to allow exhaust gas to flow into thehoneycomb filter 20 so that PMs were captured by the honeycomb filter.

Then, a relation was determined between the amount (g/L) of captured PMsper liter of an apparent volume of the honeycomb filter and the pressureloss (kPa).

FIG. 10A is a graph showing the relation between the PM capture amountand the pressure loss measured in Example 1 and Comparative Example 1.

The graph in FIG. 10A clearly shows that the initial pressure loss, i.e.the pressure lose when the PM capture amount was 0 g/L, was as low as2.22 kPa, and the pressure less was as low as 5.11 kPa even when the PMcapture amount was 6 g/L in the honeycomb filter of Example 1. Thus, thehoneycomb filter of Example 1 had a significant effect of achieving, inaddition to a lower initial pressure loss, a lower pressure loss ascompared to the honeycomb filter of Comparative Example 1 at any timewhen the PM capture amount was 0 g/L to 6 g/L. In the honeycomb filterof Comparative Example 1, the initial pressure loss, i.e. the pressureloss when the PH capture amount was 0 g/L, was 2.89 kPa, and thepressure loss was 6.15 kPa when the PM capture amount was 6 g/L.

Second Embodiment

The following will discuss a honeycomb filter according to the secondembodiment of the present invention.

The honeycomb filter according to the second embodiment of the presentinvention includes a plurality of honeycomb fired bodies having an outerwall on the periphery thereof, and the honeycomb fired bodies arecombined with one another by adhesive layers residing therebetween. Eachhoneycomb fired body includes exhaust gas emission cells each having anopen end at an exhaust gas emission side and a plugged end at an exhaustgas introduction side, and exhaust gas introduction cells each having anopen end at the exhaust gas introduction side and a plugged end at theexhaust gas emission side, the exhaust gas introduction cells includingfirst exhaust gas introduction cells and second exhaust gas introductioncells. The plugged ends are arranged in vertical and horizontal lineswith the cell walls residing therebetween in the end face at the exhaustgas emission side.

The exhaust gas emission cells are each adjacently surrounded fully bythe first exhaust gas introduction cells and the second exhaust gasintroduction cells across the porous cell walls.

In the cross section perpendicular to the longitudinal direction of thecells, each second exhaust gas introduction cell has a larger crosssectional area than each first exhaust gas introduction cell, and eachexhaust gas emission cell has the sane cross sectional area as eachsecond exhaust gas introduction cell.

In the cross section perpendicular to the longitudinal direction of thecells, the exhaust gas emission cells and the exhaust gas introductioncells are each polygonal, and a side facing one exhaust gas emissioncell among the sides forming the cross sectional shape of the firstexhaust gas introduction cell is longer than a side facing one exhaustgas emission cell among the sides forming the cross sectional shape ofthe second exhaust gas introduction cell.

In relation to the cells adjacent to the outer wall, the exhaust gasemission cells and the first exhaust gas introduction cells arealternately arranged.

The exhaust gas introduction cells and the exhaust gas emission cellseach have a uniform cross sectional shape except fox the pluggedportion, in a direction perpendicular to the longitudinal direction ofthe cells thoroughly from the end at the exhaust gas introduction sideto the end at the exhaust gas emission side.

The honeycomb filter includes silicon carbide honeycomb fired bodies.The silicon carbide honeycomb fired bodies include silicon carbidegrains having a silicon-containing oxide layer with a thickness of 0.1to 2 μm on the surface thereof. The aperture ratio of the end face atthe exhaust gas emission side is not less than 20%.

Namely, the honeycomb filter according to the second embodiment has thesame structure as that of the honeycomb filter according to the firstembodiment. They are the same in terms of the basic shape andarrangement of the cells but different in that, in the honeycomb filteraccording to the second embodiment, the cross sectional shape of thecells adjacent to the outer wall is the same as the cross sectionalshape of the cells other than the cells adjacent to the outer wall.

FIG. 11A is a perspective view schematically illustrating one example ofa honeycomb filter according to the second embodiment of the presentinvention. FIG. 11B is a perspective view illustrating a honeycomb firedbody forming the honeycomb filter.

The cross sectional shape and arrangement of the cell walls in ahoneycomb fired body included in a honeycomb filter shown in FIGS. 11Aand 11B are as follows. Exhaust gas emission cells 11 having anoctagonal cross sectional shape are each adjacently surrounded fully byfirst exhaust gas introduction cells 12 having a square cross sectionalshape and second exhaust gas introduction cells 14 having an octagonalcross sectional shape across porous cell walls. the first exhaust gasintroduction cells 12 and the second exhaust gas introduction cells 14are alternately arranged around the exhaust gas emission cells 11. Eachsecond exhaust gas introduction cell 14 has a larger cross sectionalarea than each first exhaust gas introduction cell 12. Each exhaust gasemission cell 11 has the same cross sectional area as each secondexhaust gas introduction cell 14. The honeycomb fired body 10 a has anouter wall 17 a on the periphery thereof. The cells adjacent to theouter wall 17 a include the first exhaust gas introduction cells 12 andthe exhaust gas emission cells 11.

A side 12 a facing one exhaust gas emission cell 11 among the sidesforming the cross sectional shape of the first exhaust gas introductioncell 12 is longer then a side 14 a facing one exhaust gas emission cell11 among the sides forming the cross sectional shape of the secondexhaust gas introduction cell 14.

The honeycomb fired body 10 a is different from the honeycomb fired body10 included in the honeycomb filter 20 shown in FIG. 1 in that, as shownin FIG. 11B, the exhaust gas emission cells 11 and the first exhaust gasintroduction cells 12 adjacent to the outer wall respectively have thesame cross sectional shapes as the exhaust gas emission cells 11 and thefirst exhaust gas introduction cells 12 other than the cells adjacent tothe outer wall.

The following will discuss a modified example of the honeycomb filteraccording to the second embodiment of the present invention.

FIG. 12A is a perspective view schematically illustrating one modifiedexample of a honeycomb fired body forming the honeycomb filter accordingto the second embodiment of the present invention. FIG. 12B is an endface view of the honeycomb fired body in FIG. 12A.

FIGS. 12A and 12B illustrates a honeycomb fired body 10 b forming thehoneycomb filter in which all the first exhaust gas introduction cells12 have the same shape and all the second exhaust gas introduction cells14 have the same shape. A side 170 b corresponding to an outer peripheryof an outer wall 17 b changes In accordance with the shapes of the firstexhaust gas introduction cells 12 and the exhaust gas emission cells 11adjacent to the outer wall 17 b. The thickness of the outer wall 17 b isuniform.

In other words, to make the thickness of the outer wall 17 b uniform,the side 170 b corresponding to the outer periphery if the outer wall 17b becomes rugged along with the shapes of the first exhaust gasintroduction cells 12 and the exhaust gas emission cells 11 adjacent tothe outer wall 17 b.

FIG. 12C is an end face view illustrating another modified example of ahoneycomb fired body forming the honeycomb filter according to thesecond embodiment of the present invention.

In a honeycomb fired body 10 c shown in FIG. 12C, all the cells adjacentto an outer wall 17 c are exhaust gas introduction cells. The shape of asecond exhaust gas introduction cell 14A is, compared to the shape of anexhaust gas introduction cell not adjacent to the outer wall 17 c,partially deformed to be hexagonal in accordance with the line along theinner wall forming the outer wall 17 c of first exhaust gas introductioncells 12A adjacent to the outer wall 17 c. Similarly, an exhaust gasemission cell 14B at the corner portion is partially deformed to bepentagonal.

As above, all the cells adjacent to the outer wall may be exhaust gasintroduction cells with an aim of enhancing the aperture ratio of theexhaust gas introduction cells.

The features of the embodiment other than those mentioned above are thesane as the features described in relation to the first embodiment, andthus the explanation thereof is omitted.

The honeycomb filter according to the present embodiment can bemanufactured by the sane method as described in the first embodiment ofthe present invention, except that the shape of the mold used in theextrusion molding is changed.

Being similar to the honeycomb filter 20 of the first embodiment in thebasic arrangement and shapes of the cells and the pattern of plugs, thehoneycomb filter according to the present embodiment can exert the sameeffects as the effects (1) to (10) mentioned in the first embodiment.

Third Embodiment

The following will discuss a honeycomb filter according to the thirdembodiment of the present invention. Features not described below aresubstantially the same as those in the honeycomb filter according to thefirst embodiment.

The honeycomb filter according to the third embodiment of the presentinvention includes a plurality of honeycomb fired bodies having an outerwall on the periphery thereof, and the honeycomb fired bodies arecombined with one another by adhesive layers residing therebetween. Eachhoneycomb fired body includes exhaust gas emission cells each having anopen end at an exhaust gas emission side and a plugged end at an exhaustgas introduction side, end exhaust gas introduction cells each having anopen end at the exhaust gas introduction side and a plugged end at theexhaust gas emission side, the exhaust gas introduction cells includingfirst exhaust gas introduction cells and second exhaust gas introductioncells. The plugged ends are arranged in vertical and horizontal lineswith the cell walls residing therebetween in the end face at the exhaustgas emission side.

The exhaust gas emission cells are each adjacently surrounded fully bythe first exhaust gas introduction cells and the second exhaust gasintroduction cells across the porous cell walls.

In the cross section perpendicular to the longitudinal direction of thecells, each second exhaust gas introduction cell has a larger crosssectional area than each first exhaust gas introduction call, and eachexhaust gas emission cell has the same cross sectional area as eachsecond exhaust gas introduction cell.

In the cross section perpendicular to the longitudinal direction of thecells, the exhaust gas emission cells, the first exhaust gasintroduction cells, and the second exhaust gas introduction cells areeach square, and one of the sides forming the cross sectional shape ofthe first exhaust gas introduction cell faces one exhaust gas emissioncell, and none of the sides forming the cross sectional shape of thesecond exhaust gas introduction cell faces the sides forming the exhaustgas emission cell.

The cells adjacent to the outer wall include the first exhaust gasintroduction cells and the exhaust gas emission cells.

The exhaust gas introduction cells and the exhaust gas emission cellseach have a uniform cross sectional shape except for the plugged portionin a direction perpendicular to the longitudinal direction of the cellsthoroughly from the end at the exhaust gas introduction side to the endat the exhaust gas emission side.

The honeycomb filter includes silicon carbide honeycomb fired bodies.The silicon carbide honeycomb fired bodies include silicon carbidegrains having a silicon-containing oxide layer with a thickness of 0.1to 2 μm on the surface thereof. The aperture ratio of the end face atthe exhaust gas emission side is not less than 20%.

In other words, the honeycomb filter according to the third embodimenthas substantially the same structure as the honeycomb filter accordingto the first embodiment, except that all the exhaust gas emission cells,the first exhaust gas introduction cells, and the second exhaust gasintroduction cells each have a square cross section, and also except forthe features mentioned below.

FIG. 13A is an end face view schematically illustrating one example ofthe cell arrangement in an end face of the honeycomb fired body formingthe honeycomb filter according to the third embodiment of the presentinvention. FIG. 14 is a view of an end face at the exhaust gas emissionside of the honeycomb fired body in FIG. 13A.

In a honeycomb fired body 40 included in the honeycomb filter shown inFIG. 13A, exhaust gas emission cells 41 having a square cross sectionare each adjacently surrounded fully by first exhaust gas introductioncells 42 each having a square cross section and second exhaust gasintroduction cells 44 each having a square cross section across porouscell walls therebetween. The first exhaust gas introduction cells 42 andthe second exhaust gas introduction cells 44 are alternately arrangedaround each exhaust gas emission cell 41. Each second exhaust gasintroduction cell 44 has a larger cross sectional area than each firstexhaust gas introduction cell 42, and each exhaust gas emission cell 41has the same cross sectional area as each second exhaust gasintroduction cell 44.

In the cross section of the three kinds of adjacent cells, namely, theexhaust gas emission cell 41, the second exhaust gas introduction cell44, and the first exhaust gas introduction cells 42, a side 41 a facingthe first exhaust gas introduction cell 42 across a cell wall 43 amongthe sides of the square exhaust gas emission cell 41 is parallel to aside 42 a facing the exhaust gas emission cell 41 across the cell wall43 among the sides of the square first exhaust gas introduction cell 42.

Moreover, a side 42 b facing the second exhaust gas introduction cell 44across the cell wall 43 among the sides of the first exhaust gasintroduction cell 42 is parallel to a side 44 b facing the first exhaustgas introduction cell 42 across the cell wall 43 among the sides of thesecond exhaust gas introduction cell 44. Furthermore, the distancesbetween the parallel sides of all the above pairs are the same. That is,the distance between the parallel sides 41 a and 42 a, and the distancebetween the parallel sides 42 b and 44 b, are the same.

The square exhaust gas emission cell 41 is adjacently surrounded byalternately arranged four pieces of the first square exhaust gasintroduction cells 42 and four pieces of the second square exhaust gasintroduction cells 44 across the cell walls 43. The cross sectional areaof the second exhaust gas introduction cell 44 is larger than the crosssectional area of the first exhaust gas introduction cell 42.

Furthermore, the exhaust gas emission cells 41, the first exhaust gasintroduction cells 42, and the second exhaust gas introduction cells 44are each arranged in a manner satisfying the conditions below.

Namely, among hypothetical segments connecting geometric centers ofgravity of the square shapes of the four second exhaust gas introductioncells 44 surrounding the exhaust gas emission cell 41, an intersectionof the two segments crossing the square shape region of the exhaust gasemission cell 41 is identical with the geometric center of gravity ofthe square cross section of the exhaust gas emission cell 41.

Moreover, among the hypothetical segments connecting the geometriccenters of gravity of the square shapes of the four second exhaust gasintroduction cells 44, the four segments not crossing the square shaperegion of the exhaust gas emission cell 41 forms a square, and midpointsof the respective sides of the square are identical with the geometriccenters of gravity of the respective square shapes of the four firstexhaust gas introduction cells 42 surrounding the exhaust gas emissioncell 41.

As described above, the square exhaust gas emission cell 41 isadjacently surrounded by alternately arranged four pieces of the firstsquare exhaust gas introduction cells 42 and four pieces of the secondsquare exhaust gas introduction cells 44 across the cell walls 43 toform a single unit. The unit is two-dimensionally repeated, where thefirst exhaust gas introduction cells 42 and the second exhaust gasintroduction cells 44 in the unit are shared between adjacent cellunits, to form a honeycomb filter. The units share the first exhaust gasintroduction cells 42 and the second exhaust gas introduction cells 44.Thus, the first exhaust gas introduction cell 42 and the second exhaustgas introduction cell 44, which face the exhaust gas emission cell 41across the cell walls 43, face the exhaust gas emission cell 41 in theadjacent unit across the cell wall 43.

As shown in FIG. 14, the plugged portions (portions corresponding to thefirst exhaust gas introduction cells 42 and the second exhaust gasintroduction cells 44) are arranged in vertical and horizontal lineswith the cell walls 43 residing therebetween at the exhaust gas emissionsite. The width of the plugged portions is indicated as Lc and Ld.

FIG. 15 is an enlarged cross sectional view perpendicular to thelongitudinal direction of the honeycomb filter. FIG. 15 illustrates howeach cell unit (cell structure) is two-dimensionally, i.e. in X and Ydirections shown in FIG. 15, repeated in the case where the firstexhaust gas introduction cells 42, the second exhaust gas introductioncells 44, and the exhaust gas emission cells 41 are square and theaforementioned conditions are satisfied, and also illustrates how thefirst exhaust gas introduction cells 42 and the second exhaust gasintroduction cells 44 in the unit are shared between the cell units(cell structure). A cell unit 1, a cell unit 2, and a cell unit 3 eachhave a structure in which the exhaust gas emission cell 41 is fullysurrounded by alternately arranged four pieces of the first exhaust gasintroduction cells 42 and four pieces of the second exhaust gasintroduction cells 44 across the cell walls 43 in a manner satisfyingthe aforementioned conditions.

The cell unit 2 has the same structure as that of the cell unit 1. Thecell unit 2 is adjacent to the cell unit 1 in the X direction whilesharing one piece of the first exhaust gas introduction cell 42 and twopieces of the second exhaust gas introduction cells 44 with the cellunit 1. The cells shared between the cell unit 1 and the cell unit 2 aredepicted as “shared portion 2” in FIG. 15. The cell unit 3 has the samestructure as that of the cell unit 1. The cell unit 3 is adjacent to thecell unit 1 in the Y direction while sharing one piece of the firstexhaust gas introduction cell 42 and two pieces of the second exhaustgas introduction cells 42 with the cell unit 1. The cells shared betweenthe cell unit 1 and the cell unit 3 are depicted as “shared portion 1”in FIG. 15.

Meanwhile, FIG. 15 shows four segments h, i, j, and k that do not crossthe square shape region of the exhaust gas emission cell 41, andhypothetical two segments l and m that cross the square shape region ofthe exhaust gas emission cell 41, among hypothetical segments connectingthe geometric centers of gravity of the square shapes of the four piecesof the second exhaust gas introduction cells 44. The “shared portion 2”is depicted by cross-hatching with segments in the same direction asthat of the segment m, and the “shared portion 1” is depicted bycross-hatching with segments in the same direction as that of thesegment l.

As shown in FIG. 15, an intersection of the two segments l and m isidentical with the geometric center of gravity of the exhaust gasemission cell 41.

In the cross section of the cells, one of sides forming the crosssectional shape of the first exhaust gas introduction cell 42 faces oneexhaust gas emission cell 41. Also, the second exhaust gas introductioncell 44 and the exhaust gas emission cell 41 are arranged so that theyface each other at their corner portions. Thus, none of the sidesforming the cross sectional shape of the second exhaust gas introductioncell 44 faces the sides forming the exhaust gas emission cell 41. Thecells adjacent to an outer wall 47 include the first exhaust gasintroduction cells 42 and the exhaust gas emission cells 41.

None of the sides forming the cross section of the second exhaust gasintroduction cell faces exhaust gas emission cells in the presentembodiment, and thus exhaust gas more easily flows into the firstexhaust gas introduction cells at an initial stage as compared to thefirst embodiment. For this reason, PMs accumulate earlier on the innercell walls of the first exhaust gas introduction cells corresponding tothe cell walls separating the first exhaust gas introduction cells andthe exhaust gas emission cells. Consequently, the aforementionedswitching of the main channel occurs further earlier. Therefore, PMstend to uniformly accumulate on the inner cell walls of the firstexhaust gas introduction cells and the inner cell walls of the secondexhaust gas introduction cells so that the pressure loss can be furtherreduced after accumulation of a certain amount of PMs.

The cross sectional area of each first exhaust gas introduction cell 42is preferably 20 to 50% the size, and more preferably 22 to 45% the sizeof the cross sectional area of each second exhaust gas introduction cell44.

In the honeycomb fired body shown in FIGS. 13A to 13C, the crosssectional area of each exhaust gas emission cell 41 is equal to thecross sectional area of each second exhaust gas introduction cell 44;however, the cross sectional area of each exhaust gas emission cell 41may be larger than the cross sectional area of each second exhaust gasintroduction cell 44.

The cross sectional area of each exhaust gas emission cell 41 ispreferably 1.05 to 15 times the size of the cross sectional area of eachsecond exhaust gas introduction cell 44.

The following describes the thickness of the cell walls in the crosssection of the honeycomb fired body 40 according to the third embodimentbased on the aforementioned definition of the cell walls. Supposing thata straight line Z₄₂ connecting the center of gravity O₄₁ of the exhaustgas emission cell 41 and the center of gravity O₄₂ o the first exhaustgas introduction cell 42 is given, the thickness of the cell wall 43 ata part overlapped with the straight line Z₄₂ (the thickness between theside 42 a and the side 41 a) is determined as thickness X₁. Supposingthat a straight line Z₄₄ connecting the center of gravity O₄₄ of thesecond exhaust gas introduction cell 44 and the center of gravity O₄₁ ofthe exhaust gas emission cell 41 is given, the thickness of the cellwall 43, which separates the second exhaust gas introduction cell 44 andthe exhaust gas emission cell 41, at a part overlapped with the straightline Z₄₄ (the length between a corner portion 44 c of the second exhaustgas introduction cell 44 and a corner portion 41 c of the exhaust gasemission cell 41) is determined as thickness Y₁.

The thickness of the cell walls of the honeycomb fired body 40 variesdepending on the parts as shown in FIG. 13A; however, the thicknesses,including the thickness X₁ and the thickness Y₁, may be set to be withina range of 0.10 to 0.46 mm.

The following will discuss a modified example of the honeycomb filteraccording to the third embodiment of the present invention.

FIG. 13B is an end face view illustrating one modified example of ahoneycomb fired body forming the honeycomb filter according to the thirdembodiment of the present invention.

In a honeycomb fired body 40 a shown in FIG. 13B, the shape of anexhaust gas emission cell 41A adjacent to an outer wall 47 a is,compared to the shape of an exhaust gas emission cell 41 not adjacent tothe outer wall 47 a, partially deformed to be rectangle in accordancewith the line along the inner wall forming the outer wall of the firstexhaust gas introduction cells 42A adjacent to the outer wall 47 a. Anexhaust gas emission cell 41B at the corner portion has a square shapewith a smaller cross sectional area compared to the exhaust gas emissioncell 41 not adjacent to the outer wall 47 a.

With such shapes of the cells, the boundary between the outer wall 47 aand the exhaust gas emission cells 41A and 41B and the first exhaust gasintroduction cells 42A adjacent to the outer wall 47 a is formedlinearly, and the thickness of the outer wall 47 a is uniform.

The following will discuss another modified example of the honeycombfilter according to the third embodiment of the present invention.

FIG. 13C is an end face view illustrating another modified example of ahoneycomb fired body forming the honeycomb filter according to the thirdembodiment of the present invention.

In a honeycomb fired body 40 b shown in FIG. 13C, all the first exhaustgas introduction cells have the sane shape, and all the exhaust gasemission cells 41 have the same shape. A side 470 b corresponding to anouter periphery of an outer wall 47 b changes in accordance with theshapes of the first exhaust gas introduction cells 42 and the exhaustgas emission cells 41 adjacent to the outer wall 47 b. The thickness ofthe outer wall 47 b is uniform.

In other words, to make the thickness of the outer wall 47 b uniform,the side 470 b corresponding to the outer periphery of the outer wall 47b becomes rugged along with the shapes of the first exhaust gasintroduction cells 42 and the exhaust gas emission cells 41 adjacent tothe outer wall 47 b.

The features of the embodiment other than those mentioned above are thesame as the features described in relation to the first embodiment, andthus the explanation thereof is omitted.

The honeycomb filter according to the present embodiment can bemanufactured in the same manner as in the first embodiment of thepresent invention, except that a die having a different shape is used inthe extrusion molding process.

In the honeycomb filter according to the present embodiment, unlike thefirst embodiment, the exhaust gas emission cells 41 and the secondexhaust gas introduction cells 44 have square cross sections, and allthe sides forming the cross sectional shape of the second exhaust gasintroduction cells 44 do not face the exhaust gas emission cells 41.Among the sides forming the cross sectional shape of the first exhaustgas introduction cells 42, a side 42 a faces one of the exhaust gasemission cells 41. Thus, like the honeycomb filter according to thefirst embodiment, it is considered that exhaust gas easily flows intothe first exhaust gas introduction cells 42 at an initial stage. Afteraccumulation of a certain amount of PMs, exhaust gas tends to flows intothe second exhaust gas introduction cells 44.

The honeycomb fired filter according to the present embodiment issubstantially the same as the honeycomb filter 20 according to the firstembodiment concerning basic arrangement of cells, manner of plugging,size difference among the cross sectional areas of the cells, or thelike. Thus, the honeycomb filter according to the present embodiment canexert the same effects as the effects (1) to (4) and (6) to (10)described in relation to the first embodiment.

Fourth Embodiment

The following will discuss a honeycomb filter according to the fourthembodiment of the present invention. Features not described below aresubstantially the same as those in the honeycomb filter according to thefirst embodiment.

The honeycomb filter according to the fourth embodiment of the presentinvention includes a plurality of honeycomb fired bodies having an outerwall on the periphery thereof, and the honeycomb fired bodies arecombined with one another by adhesive layers residing therebetween. Eachhoneycomb fired body includes exhaust gas emission cells each having anopen end at an exhaust gas emission side and a plugged end at an exhaustgas introduction side, and exhaust gas introduction cells each having anopen end at the exhaust gas introduction side and a plugged end at theexhaust gas emission side, the exhaust gas introduction cells includingfirst exhaust gas introduction cells and second exhaust gas introductioncells having a larger cross sectional area than each first exhaust gasintroduction cell. The plugged ends are arranged in vertical andhorizontal lines with the cell walls residing therebetween in the endface at the exhaust gas emission side.

In the honeycomb filter according to the fourth embodiment of thepresent invention, in the cross section perpendicular to thelongitudinal direction of the cells, the cross sectional area of theexhaust gas emission cells is equal in size to the cross sectional areaof the second exhaust gas introduction cells. In the cross sectionperpendicular to the longitudinal direction of the cells, the exhaustgas emission cells and the exhaust gas introduction cells are each in ashape formed by curved lines; and the exhaust gas emission cells and thesecond exhaust gas introduction cells each have a convex square crosssection formed by four outwardly curved lines, whereas the first exhaustgas introduction cells each have a concave square cross section formedby four inwardly curved lines. The cells adjacent to the outer wallinclude the exhaust gas emission cells and the first exhaust gasintroduction cells. The honeycomb filter includes silicon carbidehoneycomb fired bodies. The silicon carbide honeycomb fired bodiesinclude silicon carbide grains having a silicon-containing oxide layerwith a thickness of 0.1 to 2 μm on the surface thereof. The apertureratio of the end face at the exhaust gas emission side is not less than20%.

The honeycomb filter according to the fourth embodiment of the presentinvention has similar features as the features of the honeycomb filteraccording to the first embodiment of the present invention, except thatthe cross sectional shapes of the exhaust gas emission cells, the secondexhaust gas introduction cells, and the first exhaust gas introductioncells are different in the direction perpendicular to the longitudinaldirection of the cells.

FIG. 16 is an end face view schematically illustrating one example ofthe cell arrangement in an end face of a honeycomb fired body formingthe honeycomb filter according to a fourth embodiment of the presentinvention.

A honeycomb fired body 60 included in the honeycomb filter according tothe fourth embodiment of the present invention includes exhaust gasemission cells 61, first exhaust gas introduction cells 62, cell walls63, and second exhaust gas introduction cells 64. The exhaust gasemission cells 61 are each adjacently surrounded fully by the firstexhaust gas introduction cells 62 and the second exhaust gasintroduction cells 64 across porous cell walls 63 residing therebetween.Plugged portions (portions corresponding to the first exhaust gasintroduction cells 62 and the second exhaust gas introduction cells 64)are arranged in vertical and horizontal lines with the cell walls 63residing therebetween in the end face at the exhaust gas emission sideof the honeycomb fired body 60 (not shown in the figure).

In the honeycomb fired body shown in FIG. 16, in the cross sectionperpendicular to the longitudinal direction of the cells, the crosssectional area of each second exhaust gas introduction cell 64 is equalin size to the cross sectional area of each exhaust gas emission cell61, and the cross sectional area of each first exhaust gas introductioncells 62 is smaller than the cross sectional area of each second exhaustgas introduction cell 64. The cross sectional area of each first exhaustgas introduction cell 62 is preferably 20 to 50% the size of the crosssectional area of each second exhaust gas introduction cell 64.

The exhaust gas emission cells 61 and the second exhaust gasintroduction cells 64 each have a convex square cross section formed byfour outwardly curved lines.

FIG. 17A is an explanatory diagram schematically illustrating oneexample of the convex square cell shape. FIG. 17B is an explanatorydiagram schematically illustrating one example of the concave squarecell shape. FIG. 17C is an explanatory diagram schematicallyillustrating one example of the concave square shape in which a vertexportion is chamfered. FIG. 17D is an explanatory diagram schematicallyillustrating one example of the convex square shape in which a vertexportion is chamfered.

FIG. 17A shows a second exhaust gas introduction cell 64 having a convexsquare cross section, and a square 66 formed by connecting four vertices64 e of the second exhaust gas introduction cell 64.

In the explanation of the embodiments of the present invention, theconvex square refers to a figure that is substantially square havingfour curved sides. The sides are curved outwardly from the square formedby connecting the four vertices of the substantially square figure.

FIG. 17A shows that the sides 64 a forming the cross section of thesecond exhaust gas introduction cell 64 are curved (convex) outwardlyfrom the geometric center of gravity of the convex square toward outsidethe square 65.

Although FIG. 17A illustrates the cross section of the second exhaustgas introduction cell 64 as an example of the convex square cell shape,the cross section of the exhaust gas emission cell 61 is substantiallythe same as the cross section of the second exhaust gas introductioncell 64.

The first exhaust gas introduction cells 62 each have a concave squarecross section formed by four inwardly curved lines.

FIG. 17B shows a first exhaust gas introduction cell 62 having a concavesquare cross section, and a square 66 formed by connecting four vertices62 e of the first exhaust gas introduction cell 62.

In the explanation of the embodiments of the present invention, theconcave square refers to a figure that is substantially square havingfour curved sides. The sides are curved (concaved) inwardly from thesquare formed by connecting the four vertices of the substantiallysquare figure toward the geometric center of gravity of the concavesquare.

FIG. 17B shows that the sides 62 a forming the cross section of thefirst exhaust gas introduction cell 62 are curved (concaved) from thesquare 66 towards the geometric center of gravity of the concave square.

According to the present embodiment, the first exhaust gas introductioncells each have an acute angle portion that causes resistance byinhibiting gas flow, whereas the second exhaust gas introduction cellseach have obtuse angles that allow easy gas flow. Thus, in comparison tothe first embodiment, after only a small amount of PMs are accumulatedon the inner cell walls separating the second exhaust gas introductioncells and the exhaust gas emission cells, exhaust gas easily flows intothe first exhaust gas introduction cells. Therefore, PMs tend touniformly accumulate on the inner walls of the first exhaust gasintroduction cells and the inner walls of the second exhaust gasintroduction cells so that the pressure loss after accumulation of acertain amount of PMs can be further reduced.

In the explanation of the embodiments of the present invention, theconvex square and the concave square include shapes that are chamferedat the vertex portions thereof.

FIG. 17C shows a shape in which a side 62 a 1 and a side 62 a 2, whichare curved lines forming the concave square, are not directly connected,and the side 62 a 1 is connected to the side 62 a 2 via a chamferedportion 62 b that is chamfered with a straight line.

In the case where the curved sides forming the concave square areconnected via the chamfered portion, an intersection 62 c ofhypothetical curved lines extended respectively from the side 62 a 1 andthe side 62 a 2 as shown by a dotted line in FIG. 17C is defined as avertex.

FIG. 17D shows a shape in which a side 64 a 1 and a side 64 a 2, whichare curved lines forming the convex square, are not directly connectedeach other, and the side 62 a 1 is connected to the side 62 a 2 via achamfered portion 64 b that is chamfered by a straight line.

In the case where the curved sides forming the convex square areconnected each other via the chamfered portion, an intersection 64 c athypothetical curved lines extended respectively from the side 64 a 1 andthe side 64 a 2 as shown by a dotted line in FIG. 17D is defined as avertex.

Whether the cross section formed by curved lines is a convex square or aconcave square can be determined by hypothetically depicting a square byconnecting the vertices (intersections 62 c or intersections 64 c).

The chamfered portion is not limited to the one chamfered with astraight line, but may be one chamfered with a curved line.

In the honeycomb filter of the present embodiment, the thickness of thecell walls 63 between the first exhaust gas introduction cells 62 andthe exhaust gas emission cells 61 is smaller than the thickness of thecell walls 63 between the second exhaust gas introduction cells 64 andthe exhaust gas emission cells 61.

The following describes the thickness of the cell walls in the crosssection of the honeycomb fired body 60 according to the fourthembodiment shown in FIG. 16 based on the aforementioned definition ofthe cell walls. Supposing that a straight line Z₆₂ connecting the centerof gravity O₆₁ of the exhaust gas emission cell 61 and the center ofgravity O₆₂ of the first exhaust gas introduction cell 62 is given, thethickness of the cell wall 63 at a part overlapped with the straightline (thickness between the side 62 a and the side 61 a) is determinedas thickness X₃. Supposing that a straight line Z₆₄ connecting thecenter of gravity O₆₄ of the second exhaust gas introduction cell 64 andthe center of gravity O₆₁ of the exhaust gas emission cell 61 as given,a thickness of the cell wall 63, which separates the second exhaust gasintroduction cell 64 and the exhaust gas emission cell 61, at a partoverlapped with the straight line Z₆₄ (distance between a vertex 64 e ofthe second exhaust gas introduction cell 64 and a vertex 61 e of theexhaust gas emission cell 61) is determined as thickness Y₃.

In the honeycomb fired body 60 according to the present embodiment, thethickness X₃ of the cell wall 63 separating the first exhaust gasintroduction cell 62 and the exhaust gas emission cell 61 is smallerthan the thickness Y₃ of the cell wall 63 separating the second exhaustgas introduction cell 64 and the exhaust gas emission cell 61.

In the honeycomb fired body 60 according to the present embodiment, thethickness of the cell wall separating the first exhaust gas introductioncell 62 and the second exhaust gas introduction cell 64 may bedetermined in the same manner as the thickness of the cell wallseparating the first exhaust gas introduction cell 62 and the exhaustgas emission cell 61.

In the honeycomb fired body 60 shown in FIG. 16, the thickness of thecell wall 63 separating the first exhaust gas introduction cell 62 andthe second exhaust gas introduction cell 62 is uniform, and is equal tothe thickness of the cell wall 63 separating the first exhaust gasintroduction cell 62 and the exhaust gas emission cell 61.

In the honeycomb filter according to the fourth embodiment of thepresent invention, the thickness of the outer wall may be uniform, theshape of the first exhaust gas introduction cells adjacent to the outerwall may have the same shape as the first exhaust gas introduction cellsnot adjacent to the outer wall, and the exhaust gas emission celladjacent to the outer wall may have the same shape as the exhaust gasemission cell not adjacent to the outer wall. With an aim of making thethickness of the outer wall substantially uniform, except for the cornerportions, a side adjacent to the outer wall of the exhaust gas emissioncell may be partially deformed along with the line connecting theoutermost points on the inner walls of the first exhaust gasintroduction cells adjacent to the outer wall.

In the honeycomb filter according to the fourth embodiment of thepresent invention, the thickness of the outer wall may be uniform alongwith the shape of the exhaust gas introduction cells adjacent to theouter wall, and the shapes of the first exhaust gas introduction cellsand the exhaust gas emission cells adjacent to the outer wall may be thesame as those of the first exhaust gas introduction cells and theexhaust gas emission cells not adjacent to the outer wall, respectively.In other words, the outer wall is bending in accordance with the shapesof the first exhaust gas introduction cells and the exhaust gas emissioncells to maintain the uniform thickness in this case.

The honeycomb filter according to the present embodiment can bemanufactured in the sane manner as in the first embodiment of thepresent invention, except that a die having a different shape is used inthe extrusion molding process.

Hereinafter, the effects of the honeycomb filter according to the fourthembodiment of the present invention are listed.

The honeycomb filter described in the first embodiment of the presentinvention has a feature that the side 12 a facing the exhaust gasemission cell 11 among the sides forming the cross sectional shape ofthe first exhaust gas introduction cell 12 is larger than the side 14 afacing the exhaust gas emission cell 11 among the sides forming thecross sectional shape of the second exhaust gas introduction cell 14.Meanwhile, the honeycomb filter according to the fourth embodiment has afeature that the thickness of the cell walls separating the firstexhaust gas introduction cells and the exhaust gas emission cells issmaller than the thickness of the cell walls separating the secondexhaust gas introduction cells and the exhaust gas emission cells. Thefourth embodiment is different on the above points from the firstembodiment. Other features are substantially the same.

A smaller thickness of the cell walls may lead to easier passage ofexhaust gas through the cell walls so that the pressure loss may bereduced. Thus, the length of the sides forming the cross sections of thecells may correspond to the thickness of the cell walls separating thecells. Hence, the honeycomb filter according to the fourth embodiment ofthe present invention can exert the same effects as the effects (1) to(3) and (6) to (9) described in relation to the first embodiment.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

1. A honeycomb filter comprising: a silicon carbide honeycomb fired bodycomprising a plurality of cells through which exhaust gas is to flow andwhich include exhaust gas introduction cells and exhaust gas emissioncells, the exhaust gas introduction cells each having an open end at anexhaust gas introduction side and a plugged end at an exhaust gasemission side, the exhaust gas emission cells each having an open end atthe exhaust gas emission side and a plugged end at the exhaust gasintroduction side, the silicon carbide honeycomb fired body includingsilicon carbide grains having a silicon-containing oxide layer with athickness of 0.1 to 2 μm on surfaces of the silicon carbide grains, theexhaust gas introduction cells and the exhaust as emission cells eachhaving a uniform cross sectional shape except for a plugged portion in across section perpendicular to a longitudinal direction of the pluralityof cells thoroughly from the exhaust gas introduction side to theexhaust gas emission side; an end face with an aperture ratio of notless than 20% at the exhaust gas emission side; and porous cell wallsdefining rims of the plurality of cells, the plugged portions of theexhaust gas introduction cells being arranged in vertical and horizontallines with the porous cell walls residing between the plugged portionsin the end face at the exhaust gas emission side.
 2. The honeycombfilter according to claim 1, wherein the end face at the exhaust gasemission side has an aperture ratio of not less than 25%.
 3. Thehoneycomb filter according to claim 1, wherein the plugged portions ofthe exhaust gas introduction cells have a width of not less than 0.5 mm.4. The honeycomb filter according to claim 1, wherein each of theexhaust gas emission cells is adjacently surrounded fully by the exhaustgas introduction cells across the porous cell walls, and wherein theexhaust gas introduction cells include first exhaust gas introductioncells and second exhaust gas introduction cells, wherein each of thesecond exhaust gas introduction cells has a cross sectional area largerthan a cross sectional area of each of the first exhaust gasintroduction cells in the cross section perpendicular to thelongitudinal direction of the plurality of cells, wherein each of theexhaust gas emission cells has a cross sectional area equal to or largerthan the cross sectional area of each of the second exhaust gasintroduction cells in the cross section perpendicular to thelongitudinal direction of the plurality of cells, and wherein, in thecross section perpendicular to the longitudinal direction of theplurality of cells, the exhaust gas introduction cells and the exhaustgas emission cells each have a polygonal shape, and wherein, in thecross section perpendicular to the longitudinal direction of theplurality of cells, a side forming a cross sectional shape of each ofthe first exhaust gas introduction cells faces one of the exhaust gasemission cells, a side forming a cross sectional shape of each of thesecond exhaust gas introduction cells faces one of the exhaust gasemission cells, and the side of each of the first exhaust gasintroduction cells is longer than the side of each of the second exhaustgas introduction cells, or a side forming a cross sectional shape ofeach of the first exhaust gas introduction cells faces one of theexhaust gas emission cells, and none of sides forming a cross sectionalshape of each of the second exhaust gas introduction cells faces theexhaust gas emission cells.
 5. The honeycomb filter according to claim4, wherein, in the cross section perpendicular to the longitudinaldirection of the plurality of cells, the exhaust gas introduction cellsand the exhaust gas emission cells each have the polygonal shape, andwherein, in the cross section perpendicular to the longitudinaldirection of the plurality of cells, length of the side forming thecross sectional shape of each of the second exhaust gas introductioncells is not more than 0.8 times length of the side forming the crosssectional shape of each of the first exhaust gas introduction cells. 6.The honeycomb filter according to claim 4, wherein, in the cross sectionperpendicular to the longitudinal direction of the plurality of cells,the exhaust gas emission cells are each octagonal, the first exhaust gasintroduction cells are each square, and the second exhaust gasintroduction cells are each octagonal.
 7. The honeycomb filter accordingto claim 4, wherein, in the cross section perpendicular to thelongitudinal direction of the plurality of cells, the cross sectionalarea of each of the second exhaust gas introduction cells is equal insize to the cross sectional area of each of the exhaust gas emissioncells, and wherein the cross sectional area of each of the first exhaustgas introduction cells is 20 to 50% size of the cross sectional area ofeach of the second exhaust gas introduction cells.
 8. The honeycombfilter according to claim 6, wherein the porous cell walls separatingthe plurality of cells have a uniform thickness in any part of thehoneycomb filter.
 9. The honeycomb filter according to claim 6, wherein,in the cross section perpendicular to the longitudinal direction of theplurality of cells, the exhaust gas emission cells have octagonal crosssections, the first exhaust gas introduction cells have square crosssections, and the second exhaust gas introduction cells have octagonalcross sections, wherein, in the cross section perpendicular to thelongitudinal direction of the plurality of cells, the cross sectionalshape of each of the second exhaust gas introduction cells is congruentwith a cross sectional shape of each of the exhaust gas emission cells,and wherein, in the cross section perpendicular to the longitudinaldirection of the plurality of cells, the exhaust gas emission cells, thefirst exhaust gas introduction cells, and the second exhaust gasintroduction calls are arranged in a manner that the exhaust gasemission cells are each surrounded by alternately arranged four piecesof the first exhaust gas introduction cells and four pieces of thesecond exhaust gas introduction cells across the porous cell walls,provided that hypothetical segments connecting geometric centers ofgravity of octagonal cross sections of the four pieces of the secondexhaust gas introduction cells surrounding a reference exhaust gasemission cell of the exhaust gas emission cells are given, anintersection of two segments crossing the reference exhaust gas emissioncell is identical with a geometric center of gravity of an octagonalcross section of the reference exhaust gas emission cell, and foursegments not crossing the reference exhaust gas emission cell form asquare, and midpoints of respective sides of the square are identicalwith geometric centers of gravity of square cross sections of the fourpieces of the first exhaust gas introduction cells surrounding thereference exhaust gas emission cell, wherein, in the cross sectionperpendicular to the longitudinal direction of the plurality of cells, aside facing one of the first exhaust gas introduction cells across afirst cell wall among sides forming the cross sectional shape of one ofthe exhaust gas emission cells is parallel to a side facing one of theexhaust gas emission cells across the first cell wall among sidesforming the cross sectional shape of one of the first exhaust gasintroduction cells, wherein, in the cross section perpendicular to thelongitudinal direction of the plurality of cells, a side facing one ofthe second exhaust gas introduction cells across a second cell wallamong sides forming the cross sectional shape of one of the exhaust gasemission cells is parallel to a side facing one of the exhaust gasemission cells across the second cell wall among sides forming the crosssectional shape of one of the second exhaust gas introduction cells,wherein, in tie cross section perpendicular to the longitudinaldirection of the plurality of cells, a side facing one of the secondexhaust gas introduction cells across a third cell wall among sidesforming the cross sectional shape of one of the first exhaust gasintroduction cells is parallel to a side facing one of the first exhaustgas introduction cells across the third cell wall among sides formingthe cross sectional shape of one of the second exhaust gas introductioncells, and wherein, in the cross section perpendicular to thelongitudinal direction of the plurality of cells, distances betweenparallel sides are the same.
 10. The honeycomb filter according to claim4, wherein, in the cross section perpendicular to the longitudinaldirection of the plurality of cells, the exhaust gas emission cells, thefirst exhaust gas introduction cells, and the second exhaust gasintroduction cells are all square.
 11. The honeycomb filter according toclaim 10, wherein, in the cross section perpendicular to thelongitudinal direction of the plurality of cells, the cross sectionalarea of each of the second exhaust gas introduction cells is equal insize to the cross sectional area of each of the exhaust gas emissioncells, and wherein, in the cross section perpendicular to thelongitudinal direction of the plurality of cells, the cross sectionalarea of each of the first exhaust gas introduction cells is 20 to 50%size of the cross sectional area of each of the second exhaust gasintroduction cells.
 12. The honeycomb filter according to claim 10,wherein, in the cross section perpendicular to the longitudinaldirection of the plurality of cells, the exhaust gas emission cells havesquare cross sections, the first exhaust gas introduction cells havesquare cross sections, and the second exhaust gas introduction cellshave square cross sections, wherein, in the cross section perpendicularto the longitudinal direction of the plurality of cells, the crosssectional shape of each of the second exhaust gas introduction cells iscongruent with a cross sectional shape of each of the exhaust gasemission cells, wherein, in the cross section perpendicular to thelongitudinal direction of the plurality of cells, the exhaust gasemission cells, the first exhaust gas introduction cells, and the secondexhaust gas introduction cells are arranged in a manner that the exhaustgas emission cells are each surrounded by alternately arranged fourpieces of the first exhaust gas introduction cells and four pieces ofthe second exhaust gas introduction cells across the porous cell walls,provided that hypothetical segments connecting geometric centers ofgravity of the square cross sections of the four pieces of the secondexhaust gas introduction cells surrounding a reference exhaust gasemission cell of the exhaust gas emission cells are given, anintersection of the two segments crossing the reference exhaust gasemission cell is identical with a geometric center of gravity of asquare cross section of the reference exhaust gas emission cell, andfour segments not crossing the reference exhaust gas emission cell forma square, and midpoints of respective sides of the square are identicalwith geometric centers of gravity of the square cross sections of thefour pieces of the first exhaust gas introduction cells surrounding thereference exhaust gas emission cell, and wherein, in the cross sectionperpendicular to the longitudinal direction of the plurality of cells, aside facing one of the first exhaust gas introduction cells across afirst cell wall among sides forming the cross sectional shape of one ofthe exhaust gas emission cells is parallel to a side facing one of theexhaust gas emission cells across the first cell wall among sidesforming the cross sectional shape of one of the first exhaust gasintroduction cells, wherein, in the cross section perpendicular to thelongitudinal direction of the plurality of cells, a side facing one ofthe second exhaust gas introduction cells across a third cell wall amongsides forming the cross sectional shape of one of the first exhaust gasintroduction cells is parallel to a side facing one of the first exhaustgas introduction cells across the third cell wall among sides formingthe cross sectional shape of one of the second exhaust gas introductioncells, and wherein, in the cross section perpendicular to thelongitudinal direction of the plurality of cells, distances betweenparallel sides are the same.
 13. The honeycomb filter according to claim4, wherein, in the cross section perpendicular to the longitudinaldirection of the plurality of cells, vertex portions of the polygonalshape are formed by curved lines.
 14. The honeycomb filter according toclaim 4, wherein, in the cross section perpendicular to the longitudinaldirection of the plurality of cells, the exhaust gas emission cells, thefirst exhaust gas introduction cells, and the second exhaust gasintroduction cells are point-symmetrical polygons each having not morethan eight sides.
 15. The honeycomb filter according to claim 1,wherein, the exhaust gas introduction cells include first exhaust gasintroduction cells and second exhaust gas introduction cells each havinga cross sectional area larger than a cross sectional area of each of thefirst exhaust gas introduction cells in the cross section perpendicularto the longitudinal direction of the plurality of cells, wherein each ofthe exhaust gas emission cells has a cross sectional area equal to orlarger than the cross sectional area of each of the second exhaust gasintroduction cells in the cross section perpendicular to thelongitudinal direction of the plurality of cells, and wherein, in thecross section perpendicular to the longitudinal direction of theplurality of cells, the exhaust gas introduction cells and the exhaustgas emission cells are each in a shape formed by a curved line, whereinthe porous cell walls includes first porous cell walls separating thefirst exhaust gas introduction cells and the exhaust gas emission cells,and second porous cell walls separating the second exhaust gasintroduction cells and the exhaust gas emission cells, and wherein athickness of each of the first porous cell walls is smaller than athickness of each of the second porous cell walls.
 16. The honeycombfilter according to claim 15, wherein, in the cross sectionperpendicular to the longitudinal direction of the plurality of cells,the exhaust gas introduction cells and the exhaust gas emission cellsare each in a shape formed by a curved line, and wherein the thicknessof each of the first porous cell walls is 40 to 75% the thickness ofeach of the second porous cell walls.
 17. The honeycomb filter accordingto claim 15, wherein, in the cross section perpendicular to thelongitudinal direction of the plurality of cells, the exhaust gasemission cells and the second exhaust gas introduction cells each have aconvex square cross section formed by four outwardly curved lines, andwherein, in the cross section perpendicular to the longitudinaldirection of the plurality of cells, the first exhaust gas introductioncells each have a concave square cross section formed by four inwardlycurved lines.
 18. The honeycomb filter according to claim 15, wherein,in the cross section perpendicular to the longitudinal direction of theplurality of cells, the cross sectional area of each of the secondexhaust gas introduction cells is equal in size to the cross sectionalarea of each of the exhaust gas emission cells, and wherein, in thecross section perpendicular to the longitudinal direction of theplurality of cells, the cross sectional area of each of the firstexhaust gas introduction cells is 20 to 50% size of the cross sectionalarea of each of the second exhaust gas introduction cells.
 19. Thehoneycomb filter according to claim 4, wherein the exhaust gasintroduction cells consist only of the first exhaust gas introductioncells and the second exhaust gas introduction cells.
 20. The honeycombfilter according to claim 1, wherein the honeycomb filter comprises aplurality of honeycomb fired bodies, wherein each of the plurality ofhoneycomb fired bodies has the exhaust gas emission cells, the firstexhaust gas introduction cells, and the second exhaust gas introductioncells, wherein each of the plurality of honeycomb fired bodies has anouter wall on an outer periphery of each of the plurality of honeycombfired bodies, and wherein the plurality of honeycomb fired bodies arecombined with one another by adhesive layers residing between theplurality of honeycomb fired bodies.
 21. The honeycomb filter accordingto claim 20, wherein the outer wall has corner portions, and wherein aside, which contacts the outer wall, of each of the exhaust gasintroduction cells and the exhaust gas emission cells adjacent to theouter wall is straight and parallel to a side corresponding to an outerperiphery of the outer wall in a manner that a thickness of the outerwall is uniform except for the corner portions in the cross sectionperpendicular to the longitudinal direction of the plurality of cells.22. The honeycomb filter according to claim 1, wherein the porous cellwalls have a thickness of 0.10 to 0.46 mm.
 23. The honeycomb filteraccording to claim 1, wherein the porous cell walls have a porosity of40 to 65%.
 24. The honeycomb filter according to claim 1, wherein theporous cell walls have pores having an average pore diameter of 8 to 25μm.
 25. The honeycomb filter according to claim 2, further comprising: aperiphery coat layer formed on a periphery of the honeycomb filter. 26.The honeycomb filter according to claim 4, wherein, in the cross sectionperpendicular to the longitudinal direction of the plurality of cells,the first exhaust gas introduction cells, the second exhaust gasintroduction cells, and the exhaust gas emission cells each have auniform cross sectional shape except for the plugged portion thoroughlyfrom the exhaust gas introduction side to the exhaust gas emission side,wherein, in the cross section perpendicular to the longitudinaldirection of the plurality of cells, the cross sectional shape of eachof the first exhaust gas introduction cells is different from the crosssectional shape of each of the second exhaust gas introduction cells,and wherein, in the cross section perpendicular to the longitudinaldirection of the plurality of cells, the cross sectional shape of eachof the exhaust gas emission cells is different from the cross sectionalshape of each of the first exhaust gas introduction cells.
 27. Thehoneycomb filter according to claim 4, wherein, in the cross sectionperpendicular to the longitudinal direction of the plurality of cells, acell unit is two-dimensionally repeated in a manner that the firstexhaust gas introduction cells and the second exhaust gas introductioncells surrounding each of the exhaust gas emission cells in the cellunit are shared between adjacent cell units, wherein the cell unit has acell structure such that each of the exhaust gas emission cells isadjacently surrounded fully by the exhaust gas introduction cells acrossthe porous cell walls, the exhaust gas introduction cells includes thefirst exhaust gas introduction cells and the second exhaust gasintroduction cells, each of the second exhaust gas introduction cellshas the cross sectional area larger than the cross sectional area ofeach of the first exhaust gas introduction cells in the cross sectionperpendicular to the longitudinal direction of the plurality of cells,and each of the exhaust gas emission cells has the cross sectional areaequal to or larger than the cross sectional area of each of the secondexhaust gas introduction cells in the cross section perpendicular to thelongitudinal direction of the plurality of cells, and wherein, in thecross section perpendicular to the longitudinal direction of theplurality of cells, the exhaust gas introduction cells and the exhaustgas emission cells have one of a first structure such that the exhaustgas introduction cells and the exhaust gas emission cells each have thepolygonal shape, a side forming the cross sectional shape of each of thefirst exhaust gas introduction cells faces one of the exhaust gasemission cells, a side forming the cross sectional shape of each of thesecond exhaust gas introduction cells faces one of the exhaust gasemission cells, and the side of each of the first exhaust gasintroduction cells is longer than the side of each of the second exhaustgas introduction cells, or the exhaust gas introduction cells and theexhaust gas emission cells each the polygonal shape, a side forming thecross sectional shape of each of the first exhaust gas introductioncells faces one of the exhaust gas emission cells, and none of sidesforming the cross sectional shape of each of the second exhaust gasintroduction cells faces the exhaust gas emission cells, and a secondstructure such that the exhaust gas introduction cells and the exhaustgas emission cells are each in a shape formed by a curved line, theporous cell walls include first porous cell walls separating the firstexhaust gas introduction cells and the exhaust gas emission cells, andsecond porous cell walls separating the second exhaust gas introductioncells and the exhaust gas emission cells, and 15 a thickness of each ofthe first porous cell walls is smaller than a thickness of each of thesecond porous cell walls.